Disc brakes - useless when wet!

Being Shimano they will be mineral oil filled, which is not hygroscopic unlike dot fluid, so they are unlikely to need bleading unless they have been very poorly bled initially, which is quite a rare situation as they are all done in factory. The easy way to check is simply be lever feel, ie. is it firm?

Shimano pads are probably IMO the achiles heel of their brakes, they are hippy hugging envioment friendly organic things that just don't do as good a job as the fluffy animal poisoning evil sintered pads.

Try looking at superstar components for pads, theey are super cheap and work well.

As stated above, keep any lube or WD40 type products away from them and occasionally give them a really good yank when heading down hill fast to take off any glaze andbuild a bit of heat in them.

Sintered pads are also less succeptible to contamination than organic/fibre pads.

If they have been contaminated or glazed it's worth popping the pads out laying a sheet of of 400 Grade Wet and dry on a very flat surface and rubbing the pad in a circular motion to take of the surface layer, you can then re bed them in.

You can get back most pads by using this method

Ditto the above.... I have all sorts in the workshop with disc brake issues.... cable and hydraulic.... 9/10 is contaminated pads....but then here in the low lands of East Anglia....we do have a few hills... disc brakes are not used hard enough to prevent glazing ... with all the crap on the roads( brake and other particulates from vehicles) and discs low down the are quick to collect such matter.... 

I say to all disc users this way.... use them hard... 

well - if you do want to clean them - use only isopropyl - anything else will leave a residue

sounds like you got some oily stuff on the discs and the water lifts it into a film - I wouldn;t be too precious about cleaning them - I've had a single pair of hopes on different mountain bikes for 15 years and never had to bleed them, just wash the occasional grit out with water and leave them to themselves - they need to get a patina on them

I've only changed the pads once in that time

There is no good reason or lack of technique that would make a properly set up hydraulic brake work in the dry but not in the wet. It is either a manufacturing issue or pad/disc related. Take your bike to another workshop and pay them to sort it. It shouldn't cost much. 

The Br-M355 as a budget brake will have I believe the Resin Pads and a Resin Only Rotor from memory.

A couple of things, firstly the brake is about the cheapest hydraulic Shimano make so it's not up there in performance anyway. There is no need to "clean out the system" if you bought the bike in summer, they may have bled them but you wouldn't get good braking in the dry either if they needed bleeding.

Contamination of the pads through use of cleaning products or lubrication may be the cuase but you'd expect the pads to howl if this was the case.

How do you apply your brakes? Mostly it's down to user error if you slowly apply the brakes and gradually apply pressure this is wrong and can lead to what is called fade, this is where the oil heats up and the brakes lose pressure. You should brake in short blasts... you know like last minute, knee on the ground cornering like a boss.

Another consideration you're not some massive heffer are you? We had some 18stoner who kept saying his bike was crap so we stuck some 200/180 rotors on and told him to lay off the pies.

as another bloke said also its advisable much like in a car to every so often dry off the rotors by applying the brake in the wet

I've moved onto Kevlar pads from Gorilla brakes. Been near silent in all conditions. Haven't got the initial bite of organics, but the heat up quickly, with great profession as you pull the lever harder.

Metal/sintered or resin pads?

The metal pad type may be a better bet in the rain for outright stopping power, as long as they are bedded in properly.

Resin offer a slightly better modulation (and less noise/less longevity).

Also what type disk rotors are being used?

I find that the more solid type rotors (with less cut outs) are less effective in wet as they don't allow the excess water to be expelled as quickly. Other users may have different experiences.

It's a "swings and roundabouts" scenario...finding the right balance for you is a probably going to be a bit of trial and error.

Similar experience mine need prompting in the wet especially when it's cold and drizzle. Sometimes they are noisy as well. Have tried heavy braking and disc cleaner.
Now I just feather brakes now and then to clear discs when it's wet.

I have never experienced any fade with 4 bikes with disc brakes. Points I have learned with them. Certain pads aren't as good as others. Mainly organics are good in the dry but squeal like mad in the wet. The braking performance is still excellent. Sintered pads need a little more heat to work well but are better in the wet regarding noise. Again they work very will.
Bedding in is a few hard stops but not long dragging as this might glaze the pad surface, do short hard stops then ride to cool brakes down.
My question to you would be, is the brake lever a little spongy or has lots of travel before braking becomes affective? Disc brakes are easy to keep in good order. Clean the rotor with brake clean and clean rag after washing bike. Best practice is to remove pads before washing bike so no soapy liquid contaminates the material. Other than that I have only had to bleed one bike because of spongy lever.

they need to bed in, get them on a steep hill, get up to speed and yank em on about 30-40 times!