jonny boy,
Machine Mart should have supplied you with goods 'fit for purpose' in exchange for your £350. For them now to introduce "assessment by E/B" is irrelevant and improper as far as their contract with you is concerned. If the defect is a part that 'is adjustable' then I would expect them to do just that. Why have to refer it elsewhere? In any case, that is their problem, not yours. The item supplied by them (not E/B) to you was clearly unfit.
Seems to me that this has gone on long enough for you and you are entitled - it is your Statutory right - to either a fit replacement or a full refund (your choice). I would be back to them now, firming up a bit, demanding my right and quoting the Sale of Goods Act. They won't want to start fighting your rights established by Act of Parliament. After all, you have given them a fair chance to rectify the situation, and what use/value are you getting from your £350 so far? Yep, I think I would by now be getting a bit fed up and I'd be taking a firm stance on this. Otherwise, it could go on for weeks, then getting a still duff machine back, only to have to start all over again with Machine Mart. The more I think about it, I feel I would demand a full refund now, accepting nothing less, and then buy elsewhere - the machine is readily available. And, after all, they could do the same to you again.
(Sorry to rant on a bit. I do get annoyed when firms see fit to treat their customers like this. But the advice stands good!).
Good luck with sorting the problem.
Cheers,
Trev.