Well, in the end i ran memtest on each stick separately (both times using DIMM slot 1) for well over 8 hours each and not a single error with either stick...
and that's at the 2.5-3-3-6 timings..... so it's either the 2nd dimm slot that's dodgy or the fact it doesnt like running them both at the same time... ill try the 2nd dimm slot tonight.