Hi Chris,
Many thanks for the very detailed reply, you are a star !
The tool you describe sounds a bit like a hand made rug tool I have (its a small hook on a handle basically), I assume that this is just used to get the O ring (at the back of the white plastic housing) out, not to put it in... am I right? Otherwise I'll have to contact AL as you say.
My local plumbers had a tool they recommended for this that looked (to me) a bit like pliers. I couldn't see how that would help. I'll go and find the name of it, perhaps it would work.....
I have another AL mixer that's leaking (a manual model 409) that has had very little use in an en suite bathroom. And its started leaking. I think I may have installed that in 2002 or 2003 possibly - I suppose I need a new cart for that one too ! I'll be ruined at this rate.... more so !
Thanks again for everyone's help,
Tom