I'm not sure if worn 'O' rings inside the cartridge can cause the leaking valve problem, but for those who want to try I have managed to disassemble a grey (022801) Aqualisa thermostatic cartridge to gain access to the 'O' rings inside. First (after turning off supplies) remove the cartridge from the wall housing by pulling off flow knob, 4 screws on temp ring and 4 screws holding in cartridge. Undo the black turretted ring around the splined knob that the flow control cover pushes onto. I used a pair of long nosed pliers, a leg in a notch each side and turned anti-clockwise to remove. Behind this is a white ceramic/PTFE looking washer, which is easily removed. The grey coloured ring does not need to be removed. Then twist and pull the black splined piece to remove it. Then you can push out the white spiral ended central shaft by pushing something undamaging, like a pencil, through the metal cylinder from the rear end. You should now see the 3 'O' rings on this plastic shaft and they can be recoated in silicone grease. I have not yet looked for replacement items for these, two seem to be a stepped type of profile, not just your average donut shape. Am going to see if silicone grease addition will cure the leak first.
Re-assembly is the reverse process, the only tricky bit is getting theblack splined piece back in with the white shaft to go right in. The white shaft's disc end has two notches which need to line up with the corresponding lumps in the cartridge recess. If you just push the disc end far enough so it locates over these lumps, you should find the black knob will twist right into the housing with the white shaft. Only the splined part should protrude. If you can see the hollow bits, it's not far enough in. Then just replace the washer and the end ring can be tightened up again, before the cartridge is replaced in to the housing and all screws retightened, ensuring the ring gasket is properly positioned at the rear of the cartridge body to form a good seal with the wall housing.
Hope this helps. If anyone believes these seals rather than cracks in the cartridge can cause the problem then it would be nice to be able to get hold of the 'O' rings themselves and replace these rather than just a re-grease, perhaps from Aqualisa? Cheaper and greener than £70+ for a new cartridge.