right heres an update, its fixed, no noises and cost less than £10...........
it wasn't the gearbox or clutch, i popped under the truck to check the uj's as rick suggested and noticed the bracket for the exhaust had snapped, nipped to halfords got a new bracket and popped it on, no more noise, i think what was happeneing was in revers as low revs it was causing the exhaust to vibrate on the drive shaft which was sending the noise down to the gearbox making it sound like it was coming from there :doh
have had a good few reverses and all seems ok, while i was at it i did th clutch slip test and that was fine too, however after this little episode i will be doing a gear oil change in the near future :lol