I can't advise properly OP without seeing the contract.
But in summary:
TUPE probably didn't actually apply to transfer the recognition agreement - it will only have done this if your undertaking retained its identity after the transfer distinct from the rest of the new employer (this is a broad summary of a complex point). It doesn't sound like this is the case.
Your new employer might have voluntarily recognised (but they don't seem to accept this so its unlikely)
The recognition agreement is unlikely to be incorporated into your contract even if it did transfer (although it might have been - you'd need to read your contract to say whether the recognition agreement is listed as a collective agreement which has been incorporated). Collective agreements are not automatically incorporated.
Even if it is incorporated it would then need to specifically refer to you having a right to be accompanied at any meeting not just those where you have a statutory right to be accompanied - the standard recognition agreement doesn't contain this wording and so it would need to have been specifically negotiated in and expressly set out. It is likely to have wording in about the employer negotiating with the union (but not being bound to reach agreement)
Even if expressly incorporated and containing the right wording it would only be a breach of contract. This in itself is not necessarily something which gives you much bargaining power.
There is no statutory right to be accompanied at this sort of meeting.
Upshot - you are highly unlikely to have any legal right (statutory or contractual) to be accompanied. If your union won't add their weight to the argument so that the employer agrees just to get rid of the hassle and the employer won't budge then there is not much you can do apart from object. If they ultimately dismiss you could try to argue that although you have no contractual right it affected the fairness of any dismissal. Whether you are successful on that would depend on how much difference it would make to have representation - likelihood is that it wouldn't actually make much difference but you never know.