Ok... I've replaced the rear brake pads cos they were getting close to end of life but now the brakes are very poor and feel like they are made from cheese.

Basically there is no bite in them no matter how hard I push down on the brake lever. Does any one have any ideas as to what could be the problem? Should I replace the disk at the same time?

Are the pistons seized?

What is the fluid level/condition like?

seems strange that a new pad is not biting.  I would too have said seized piston if you hadn't already said its not the problem.  Did you put a pad on each side?

Maybe the disc has worn thinner, but I wouldn't have thought so (and new pads would still grip better than your old ones.) There's no blockage/air/ kink in the pipes?

The new pads have to bed into the disc, May take a a couple of hundered miles before they bed into the profile of the disc.

A bit like scrubbing new tyres. Did you allow the pads or discs to be contaminated with brake fluid?

 When did you last bleed your brakes?
 

Thanks for the replies. 

It was just a disc change, no fluid change or anything.

I've been out and about for a while now and they seem to be improving slightly and they do actually have some stopping power again so maybe it is just bedding into the discs. I'll see how they go.

I don't use the rear alot anyway - mainly for stability at slow speeds.