I've just bled my brakes (hayes stroker trials) and when I've put the wheel back in and pulled the brak, I noticed that only one of the pistons was moving. I got a ring spanner and held the free-moving piston whilst pumping the brake and the other moves slowly and is hard to push back with the spanner. What's the next step for me, does it mean taking the piston out, cleaning, then starting again?

lube with silicon grease or red brake grease, if possible work in and out till smooth again

Thanks, will I have to take the piston out to do this?

No, just pump it out a bit from the lever. Not too far, don't remove it.

Where can I get any of these greases from?

Brake fluid might do – its not as good a lube tho

Got my tub from here http://www.mgocaccessories.co.uk/acatalog/MGOC_Accessories__Grease_176.html
Trust me when I say that, if you only use it for brake seals, this will be the only time you buy a tub in your lifetime!!! Came in a proper Castrol tub, but the photo doesn't appear to show that now….

Motorcycle shops sometimes sell small amounts

Make sure you buy the right grease though, as DOT fluid will eat normal grease.

Hunters Silicone Lube, from plumbers

Copper grease (Coppaslip) works fine too.