Update.
Well after lots of reading up & listening to you-tube clips I've bought a new back box
I spoke to Turbo Thomas again & he was very helpful but is also busy moving premises & didn't seem sure when he could make one for me.
I then got chance of a Designtek box & I couldn't resist.
Not cheap but quaility is superb.
I removed the old ( original ) box without too much hassle & the new one fitted up easily, I just needed to modify one of the straps to get a good fit.
Went for a run out & I'm very happy with the sound.
At tickover & driving around town at under 4000 rpm it's not noisy at all, just a deeper burble but once over 4000 it's sounds superb.
I didn't want it too noisy when driving with the roof down & for me it's just about right, although it will probably be too quiet for some.
Thanks for the replies, Phil.