Hello Marie, I'm Keith and happy to help you with your question.
Well it is true that under UK Contract Law once an offer had been made and accepted a contract exists between the two parties. One could therefore advance the argument HMRC had offered a settlement, once the other side had accepted same, then the agreement could not be reneged upon. One would have to look very carefully at the terms of the offer to ensure that there was no loophole permitting the creditor, HMRC, to change their stance. Of course, it works the other way too if it is you who have made the offer and HMRC accepted.
I do hope my answer helps to put your mind at rest. What the HMRC official told you is, on the face of it, correct, but, as I said, look very carefully at the terms in which the offer is expressed.