Having noticed slight, then increasingly prominent water leak from the top of the group I decided to replace the piston seals. Once I removed the piston and saw what colour it was I thought ok, I'll clean and do all the seals on the group. Then I thought hmmm maybe I ought to do the seals in the steam valve too....and a new steam arm wouldn't hurt either... so to cut a long story short, I've ended up striping this machine right back to the frame and built it back up over the last couple of months.
Once it was all back together and I first switched it on there was a leak from the inside at the back of the steam valve. I replaced the recommended 'upgrade' plastic washer with a brass crush washer and that solved it straight away although there was quite a bit of water in the machine. Stripped down the power switch, dried off, cleaned and put back together then moped up remaining water and made a very acceptable espresso.
Next morning I switched the machine on, waited for it to warm up then almost finished pulling a shot when there was a bang and a blue flash (the power light remained on). Obviously I switched off and after testing the components with a metre it looks like the 'ready light' bulb and Pressurestat no longer work. I have ordered these parts but don't want a repeat - my guess is the water shorted out the light and took the Pstat with it. I followed the 'Olympia Cremina Electical Troubleshooting Guide' on the Orphanespresso site which was useful although I'm not getting expected readings from the element. Any advice from someone who actually understands electrics would be good.