Rear hydraulic disc sticking My 2012 Model Scott Sub 10 has recently been experiencing issues with the rear disc sticking on my daily commute and getting so bad that I have to stop, wait for it to cool and then push the disc a bit to give it some space. I believe this has been caused by the terrible weather that the UK has experiencing, perhaps with some debris becoming lodged in one of the pistons. 

I decided to take the pads out this evening following the guide that came with the bike and cleaned any dirt around the pads but there is still some friction there.

I am debating pouring warm/hot water over the whole brake system in an attempt to remove anything that may still be lodged. 

What advice would you you suggest aside taking it to my local bike shop?
The advice I have seen for a stuck piston is  a) hot soapy water, followed by Isopropyl Alcohol, followed by a small amount of brake fluid (Use the same that is in you brakes - mineral in your case) to lubricate the piston. If you google for "Stuck Bicycle Disc Brake piston" there is quite a lot of useful advise

Warm soapy water on it's own cannot hurt. Although I have never needed to do this, I would use a syringe to direct the water at the pistons to waste away as much crud as possible. Move pistons out (being extremely careful not to drive them out too far - put a pair of old pads in before squeezing the lever... When pushing them back in, do it gently) Watch for a piston moving more than the other. If one is not moving, hold the other in to get it free.  Then squirt water over them and wipe them off with a clean rag. 

If the crud contains too much in the way of oils, the water may not be enough  - resort to the IsoP. 

Wipe a clean rag soaked in a small amount of brake fluid over the pistons. If desperate, a normal mineral oil, but what every you do, don't use too much. You need just enough to put a thin layer over the piston, and more and you risk getting it on the pads (very bad for them). 

Finally, and most importantly, get a clean dry rag and wipe the whole assembly down as clean and shiny as you can. Install pads and see how you got on.