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!

Next time you've got the car properly warmed up and ready for some smart acceleration, you should stop and do the 10 second reset thing, and then instantly accelerate away a few times (i.e the reverse of bedding in new brakes) so that the adaptations for good performance will occur.