It worked!
I did exactly as you said above, they were the heavy duty circlip pliers, which I ground down a bit for a snugger fit.
I've been wanting to get this apart for ages, finally put a new seal in and it's upped the power a bit so that's good.
Thanks for the advice.
Do these take a standard sized spring? would be nice to fit a new spring too.
Cheers,
Matt.