Quick update:
Had a reply from Sanspeed who suggest that as they have sold over 50 without problem that it could be a dodgy MAF, air leak, or just need a remap to adjust MAF scaling. But they've also offered a refund if I'd prefer and this is what I'm going to do, as I don't want to have to pay another £300-£500 for a remap, then modify my recently renewed insurance policy just for a bit of noise and for the car to work how it should. As it's my daily driver and I'm self-employed, I can't afford to be left stranded or have the worry of that
Whilst one of my options was to destroy it with prejudice (assuming there was no offer of a refund), my other was to send it to exdos to play with
I might take it out again for one or two more runs, to video the 'flat spot/surging' and the cut-outs - and then do the same once I've put the OEM box back on!
Had a reply from Sanspeed who suggest that as they have sold over 50 without problem that it could be a dodgy MAF, air leak, or just need a remap to adjust MAF scaling. But they've also offered a refund if I'd prefer and this is what I'm going to do, as I don't want to have to pay another £300-£500 for a remap, then modify my recently renewed insurance policy just for a bit of noise and for the car to work how it should. As it's my daily driver and I'm self-employed, I can't afford to be left stranded or have the worry of that

Whilst one of my options was to destroy it with prejudice (assuming there was no offer of a refund), my other was to send it to exdos to play with

I might take it out again for one or two more runs, to video the 'flat spot/surging' and the cut-outs - and then do the same once I've put the OEM box back on!