So I put a socket in today and tried a couple of different op-amps (including the OPA134) to no avail ... in each case I just got over "overload", it looks like I was getting oscillation on the feedback pin.
Eventually I put back the original (still without the dual fet, and with bridges) and it's back to working ... it actually seems pretty good!
The relay for the AC+DC range is definitely not working, and they also seem difficult to get in the UK, so I may have to get creative on that one.
I actually think I may have damaged the dual-fet while I was trying to diagnose the problem ... thinking back, right at the start I did get good AC readings on ACV, but didn't get anything reliable on ACV+DCV ... after a bit of diagnostics I was just getting overflow, so I suspect I damaged it then.
So again I'm a bit stuck ... do I just leave it without the FET's? What's the implication? It seems to be reading as well as my Keithley 2000? Or do I try to find some FET's to put in there ... if so, where do I start!
(And if anyone knows a source for the relays in the UK, or a sensible alternative then please let me know (0490-0683)
Edit: How about these?
http://uk.farnell.com/cynergy3/s2-05e/relay-reed-spst-no-350vdc-1a-tht/dp/2480006 They should fit reasonably well, with only one lead needing to be messed with.
Thanks,
Lee.