Thanks Gents!
Sadly the contacts on these relays are recessed so the paper trick doesn't fit

The contact surfaces are rare metal plated so I don't want to be too harsh with them - there seems to be a powdery deposit on some of them. Possibly oxide but I don't think so. I did another relay dismantle this afternoon, this time one with an auxiliary contact set added on the top. Doing the relay was just like the last one, but I could find no way of getting at the auxiliary contacts short of busting the plastic moulding. The contacts were bad, which is why I pulled it out, but I had in the end to try and flood the inside with contact cleaner / de-oxidiser and even getting that 'inside' was flaky - obviously essentially sealed. However working it by hand with the switch cleaner floating around in the end gave a good contact, but not a satisfactory solution really.
Meanwhile if anyone knows a source of Siemens relays at a sensible price:
Main relay 3TH 2031-0BB4 (3 N/O & 1 N/C 24v DC coil)
Aux Block 3TX442-2A (2 N/O & 2 N/C)
please let me know. They need really to have the same numbering scheme as the following pictures - the are a few around marked up L1 L2 L3 but I feel that is just going to lead to confusion in the future