Hi all, well I finally cracked it today!
Bench tested starter again today and worked fine. I must have not done it correctly yesterday.
So, more confused than ever but with a fresh mind and multimeter in hand I checked, the wireing to glow plug relay including in line fuse - all fine. Wire down to glow plug rail -fine. Checked the resistace on both under car earth straps fine. Wireing to starter motor - fine but still it was not pipping in and I couldn't turn the engine over using the shorting method - which made me think earth again! With ignition off I still had 12.7 volts between enginge and positive battery terminal. When I turned the ignition this dropped and was all over the place. So I was just about to give in and spit the dummy out when I asked my father to try and crank the car whilst I gave the wiring a good wiggle in different areas.
No luck under the bonnet or in the engine bay. So I slid under the car and wiggled the earth straps, I got a big spark from the end of the crimped part of the actual cable to the bolt that was holding it to the car. So took the strap off sanded either end plus around the nut and 'hey presto' she sparked into life! - RESULT!
When I'd checked resistance of the strap I'd poked the probes of the mulitimeter into the cable rather than on the rings at either end, bypassing the fault and getting a good reading

. So To summarise I guess I must of caught the cable whilst replacing the exhaust and distubed the connection.
Thanks to everyone who offered thier help, if there's any of our accessories you need, drop me a line and I'll sort some healty discounts for you
Cheers all,
Nick