There are plenty of explanations about removing this shower controller cartridge (both the thermostatic version - 605 and the manual version - 405) but nowhere could I find out how to dismantle the cartridge so that I could replace the O rings inside.
I believe that a whole range of Aqualisa cartridges are very similar.
This tiny instructable, simply shows how to open the controller so that the O rings can be replaced. A set of six O rings can be purchased on eBay (£7.95 July 2014) this might save you paying £90 for a replacement cartridge.
The eBay supplier also offers a set of O rings which includes the 'main' gasket - mine was in good condition - this enhanced set currently costs £14.95. This gasket can be seen in the first photo.
The process is trivial (once you know which bit opens). My controller had been installed for nearly 20 years, and the process required hardly any force to achieve.
BEWARE - look at step 3 before you reassemble the control - you have been warned!
Step 1: Unscrew the 'hidden' Nut
The photos show which part unscrews. As can be seen, I just used a pair of long-nosed pliers.
Step 2: Exploded View
Once the 'nut' has been removed, the central rod can be pulled out without any more fuss. It may be a bit stiff to come out, depending on how worn the O rings are!
The replacement O rings fit on to the central rod.
Hope this helps.
Step 3: Avoid Disaster!
All went well. I Even bent a large needle into a hook shape so that I could remove the O ring right at the back of the shower housing. Putting the replacement ring back in was easy with a bright torch and a screwdriver - hang the new O ring on the tip of the screwdriver and introduce it into the housing. Push the ring into the top of its seating channel, hold it firmly in place with the screwdriver blade and push the bottom of the ring into place with a little finger - it seated itself nicely.
Final assembly was a disaster. So easy to avoid but impossible to restore once disaster has occurred - I had to buy a replacement cartridge from Ebay (I was lucky, I found one at £65 instead of £90 - July 2014).
The photos show the result of overtightening the final plastic nut - the one that this Instructible is all about!
I held it with my long-nosed pliers and tightened it up, but it didn't seem to seat fully, so I gave it a little bit more and it cracked. I was aware that it was plastic so was pretty gentle, but disaster happened anyway.
I would advise virtually no more tightening than hand tight.
If, like me, it doesn't seem to seat fully, pull out the central rod and look very carefully at the two 'cutouts' in the white plastic 'Archimedian screw bit'. Those two cutouts, must locate with the matching lugs in the black plastic housing. Its all a bit of a fiddle, but it becomes obvious, when you firmly push home the central rod and it then seats fully. The final white plastic washer will then be seating in a sort of groove which is formed, and the final plastic nut screws on fully, with hand tight force only!
The black plastic nut has no part to play in making the control water-tight (the O rings do all that) - it has very little force on it when in use.
Be warned - you can't repair the nut once it has broken (although, I could make a new one out of aluminium on my lathe ........)