You can borrow up to 50% of the net SIPP value within the pension. As an example, if you had a SIPP value of £400,000 then you could borrow £200,000.
However, if you had a SIPP with assets of £400,000 with existing borrowings of £100,000 then you could only borrow a further £50,000. This is because the net value is £300,000 and so the maximum that can be borrowed is £150,000 altogether.