Dell 2950 Memory

Differing brands rarely cause issues, IF they are otherwise identical (timings, layout, speed, etc.).  As long as the specs are the same, you can replace it without issue.

As this rarely can be an issue would it not be good practice to only install matched pairs (from the same manufacturer) in business critical machines. The reason I am asking this question is that dell have supplied me with the single dimm as a replacement when I expected them to replace the pair or at least replace with a like item.

On the other hand  I find getting ram for servers can be risky even if the ram has the same specs. Numerious times I have ordered ram from major distributors like crucial, supposed same specs, and it does not work.

I try getting the exact same modules, or if going to larger capacity sticks, I get try to get the sticks recommended by the server supplier. On another note, raid adapter cache ram can be even more tricky; last time I ordered a larger module I had to return two orders of three.

"I currently have a 2GB DIM with identical technical specification but from a different manufacturer.Can this be used to replace the faulty set"

Try it off hours, if it boots, stays up for a couple hours without issues your generally OK. Only a couple times did I have issues which reared after a week (infrequent lockups, same with blue screens). After you add the ram run the Dell Diags

Dell "certifies" memory modules to be compatible to avoid those "rare" occurrences, so regardless of which manufacturer they get the memory from, they will work without issue, as warrantied by Dell.  You didn't state earlier that it was a Dell replacement.  Doing it yourself with after-market memory, all bets are off and you go back to the "rare" but possible.