Real world BTL mortgage renewal, who should pay?

If the rent is already at market rent level then no you shouldn’t try to increase it. If it is currently below market rent then I would suggest increasing it at the next rent review date.

As has already been said, it depends how much rent you are charging.
If you’re already at market rent you have been doing very nicely up to now. If it’s below market rent your tenant (or the government) has been doing very nicely.

In reality you need to weigh up how much of an increase is appropriate (if any).
Is the rent you charge above or below LHA? Does the tenant qualify for UC? If so increasing the rent to LHA actually wouldn’t cost them a penny. The UC would increase correspondingly.

Are they fully self financing? Have they had regular pay rises? Remember minimum wage has increased massively over the last few years. People earning more than minimum wage have often had lower pay rises.

There is no ‘one size fits all’ answer.

Well Jim if you have a BTL mortgage of £54 or £144 then you should be working at paying this off ASAP
The fees and other costs to remortgage this tiny debt would mean clearing the debt makes sense.
Depending on your circumstances then you can decide to carry on being a Landlord or sell up

Thanks for the replies everyone.
I am just stunned that my payments have risen to almost 3 times what they were 5 years ago, I cannot imagine the stress this is causing anyone that has a “real”mortgage of any kind in the UK.