As above really – have had a constant ache on the back of my knee. Have rested it for a few days, but it's still there. Anyone else had something similar?

Patella tendonitis? I had it for a while. Ached most when just sitting still, and more of a stabbing pain under strain. There're various stretching excercises you can do to strengthen your quad muscles. Support bands work if you can get them to fit properly and plenty of ice packs too when not excercising.

I guess I should add:

Go see a doctor.

That sounds exactly like it. Did it naturally go after a while? Any idea what causes it? I haven't been dping anything out of the ordinary.

Apparently it's weak quads that cause it. This leads to swelling and possible infection of the cartilage. I saw a doctor about it who recommend various excercises, which I wasn't very strict about, and it seemed to go away by itself after a couple of months. It came back with a vengeance a couple of years later and had me doing regular excercises, wearing supports and icing it constantly. It may just be a coincidence but I changed to S-S just after too and I've never had a problem in about 4yrs now (probably quite an extreme step, but you do get some beefy legs).

A doctor or, more likely, a physio would be able to recommend the right excercises.

I've had similar it was a loose SPD cleat that caused mine and also badly setup SPDs.

Low seat & long hills brings mine on, just in my weak leg though.

Agree with the saddle height comment. Make sure your seat is not too low/high

Had this too recently.

Back of the knee and near the hamstring. Are you talking back of knee as back of leg or behind the kneecap?

Strengthing the quad holds the cap better but not the back of the knee.

My fault for riding too low, upped my seatpost but still too low but I forgot about the layback! so my saddle was further back even though less stress on the front of the knees.

Will move my saddle forward a bit with a plumb line etc knee and ball of foot and cheers for spd tip-mine feel loose and need tightening up.

Have you done any changes before the pain Soup?

Avoid anything that aggravates the injury.
Swimming is usually a good alternative to maintain the strength in the leg without bearing weight or impact.
If its a knee cap tracking problem – you may have a tight ITB muscle combined with a weak VMO muscle. Stretch the ITB and strengthen the VMO.
Try some quad strengthening excercises too.

Have you moved your cleats recently or checked if they have moved , that's what caused mine but it took six months and a few hundred pounds at the physio before i noticed

are you sat on your wife's lap?