I have managed to solve the problem completely but not entirely sure how!
Firstly I changed my graphics card settings and switched of some of the features that can slow performance (sacrificing quality)
Then I lowered the memory usage setting in the options of the technic launcher.
THEN when I loaded Tekkit it wouldn't let me log in to my own server saying I didn't have source forge installed!!!! Arghhh! So I deleted the .techniclauncher folder and re-ran the technic launcher update to re-install.
When I restarted Tekkit it logged me in and now works crash free for hours!
It's weird and I don't know what part of this sorted it but I'm wondering if it doesn't unpack well when installing so you need to do it a few times until all of the mods work properly???
Hope this helps one of you guys cos I know it's miserable when it's constantly crashing.