For the benefit of nyrnu, and indeed anyone else in the same position, it is worth referring to :-
Statutory Instrument 2003 No. 653 - The Tax Credits (Immigration) Regulations 2003
These are just one of the documents including detailed regulations concerning Tax Credits which were issued around the time that the credits were introduced back in 2003.
There will be quite a number of people in the same situation as nyrnu. Please refer to Regulation 3(2). In a nutshell this says that if one of a couple is subject to immigration control and the other is not, then for the purpose of tax credits only, both of them are to be treated as not being subject to immigration control. In other words, there is absolutely no problem in that couple claiming tax credits, and the claim by the person who is subject to immigration control is not a breach of the "No recourse to Public Funds" visa restriction.
Not only that, just after the regulations were published in 2003, I got an MP to ask a Parliamentary Written Question, namely :-
Mr. Simon: To ask the Secretary of State for the Home Department whether a claim under the terms of Regulation 3(2) of The Tax Credits (Immigration) Regulations 2003 (S.I. 2003, No. 653, by a couple including a person who is subject to immigration control will not be considered to be a breach of any restriction contained in a visa. [104441]
Beverley Hughes: The provisions of Regulation 3(2) of The Tax Credits (Immigration) Regulations 2003 will be fully taken into account when any changes are made to the list of public funds given at paragraph 6 of the Immigration Rules. In line with this regulation, receipt of tax credits by a couple, including one member who is a person subject to immigration control, will not be regarded as a breach of any condition of leave to enter or remain.
There there we have it, in black and white from the immigration minister at the time, in circumstances covered by regulation 3(2), there is no breach of the visa condition by the person who is subject to immigration control.
So nyrnu, nothing to do ... no need to worry!