How I understand this is that you are an employee of Company A and should have a contract of employment with them. Company A has sold your services to Company B and there will be a contract for services between them. This is perfectly normal for any contractor.

The contract between company A and B may specify the hours you are expected to work. If you are being asked to work additional hours beyond this then tell your employeer (company A) as it seems likely that in this case Company B are failing to pay for all services provided. You may find that you are legally required to do this and not doing so could lead to you being open to legal repercussions due to falsifying your timesheet. In this case as the request is at the behest of Company B they would have no right to refuse to approve the extra hours on the timesheet and you should expect company A to pay you for those hours.

On the otherhand if you choose to work those hours without being asked then you should still include them on your timesheet. In this case Company B may refuse to authorise them - in this case stop volunteering your time for nothing.

In short the max (and min) hours you are expected to work should be spellt out in the contract between companies A and B - this will dictate what company A charges company B.

I would expect your contract with company A will also have clauses relating to working time and you need to check this as it will affect what you may be recompensed - it should also detail your entitlements to Holiday and sick pay. Your first step should be to discuss this with your HR department - that is Company A.