Will a 10 speed Campag mech work on an 8 speed cassette?

It seems it is the end of the road for my 8 speed Record Titanium mech, as there is a substantial amount of play from the bolt & spring that connects the body & the cages, causing my mech to rub against my spokes when I climb in my lowest gear. I've disassembled, cleaned, re-greased & re-assembled & there is still substantial play.

I can source 8 speed mech's from eBay, but I don't know if they are going to be close to the same point that mine is now in.
If I were to get a modern 10 speed Record carbon mech, will this work with my 8 speed Ergo shifters & cassette?

I'm sorry that I'm not answering your question, but what free hub body are you running? I cannot seem to find 8 speed free hub bodies anywhere.
Sincerely hope that your question gets answered, though

If I were to get a modern 10 speed Record carbon mech, will this work with my 8 speed Ergo shifters & cassette?

I'm not sure if it will. Campagnolo changed something about the way their derailleurs function around the time of the milleneum. something to do with 'angles' or wotnot.

I have used a 9 speed rear mech with 10 speed ergos and didn't experience any problems but then again they are relatively close to each other. however 8 speed to nowadays derailleurs unless you have a free/cheap one to experiment with I wouldn't risk the money.

8 speed Record is great but one really does feel the 'clunk' factor compared to the 9-10-11 speed systems.

Fignon is right - the 8s RDs had different geometry to the 9 & 10s gears so whilst 9 & 10s RDs are basically interchangeable, the "8s" can't be replaced with a 9 or 10s. The change was made in MY2001.

You need a Campag RD with a "B" screw positioned on the mounting plate in much the same way as Shimano.

If you are after NoS, try http://www.campoldy.com - Nigel Scott usually has stock ...

1997-2000 9 speed will work, it's the 2001-6 that are different.

As mentioned above, look for the position of the B-screw: it needs to be in the same position as on your 8 speed RD.

It depends what you mean by 'work'. It'll shift gears okay but it wouldn't necessarily be clean or crisp shifting. I've ran Dura Ace/Chorus 10 speed on 8 speed cassettes during the winter but it's not been really precise or smooth. It may take quite a bit of fettling but it's possible

You can find low end Avanti RD on ebay most of the time and they are surprisingly good quality, and cheap as chips. Even nos.

If its any help, I had some play in my 8 speed mech and I made a shim to place on the pivot bolt that attaches to the dropout hanger and that got rid of all the play. Sounds like the play in yours is coming from the other end of though? Happy to pop a shim in the post if you want to try it, it might tighten it up enough to work ...