Belgian Pension

My understanding is yes, the UK is now responsible for your pension.
There is an agreement of equal responsibility by EU governments.
I may well be wrong and hope that others have accurate information.

The situation is potentially complicated because under the EU arrangements you claim your "legal" retirement pension from the country in which you reach that country's retirement age - and that "legal" pension includes not only the pension that you earned in that country but any "legal" pensions that you had earned in other EU country in previous employment - presumably in your case a UK state pension. Once you are being paid your "legal" pension my understanding is that it continues to be paid by the same country - and certainly both our state pensions are paid from the UK although we live, but have never worked, in Belgium.

I think that perhaps the critical questions are have you reached the retirement age in both the UK and Belgium, are you entitled to "legal" (i.e. state) pensions in both countries and, if so, did you declare your UK entitlement when you claimed your Belgian pension. The UK pensions people in Newcastle are quite knowledgable on the subject - IF you can get hold of the right person. There is quite a lot of information on their website (make sure it ends .gov.uk) but probably not the answer to your precise situation.

Europe Direct might be able to help you. Send your question to them.
Website link is http://europa.eu/contact/write-to-us/index_en.htm

I'd agree with BECASSE. You need to speak with the National Insurance office that deals with expat issues in Newcastle. They are extremely helpful. My parents lived, worked and then retired in Belgium. They eventually moved back to the U.K. The Newcastle office was very helpful, and the ONP (the french version of the RVP were also (surprisingly) extremely helpful and easy to deal with.