I would advise that when you serve the Subject Access Rights Notice enclose the full #10 maximum at that time. If it turns out that the fee is less (Unlikely - Why charge less when they can get #10!) then it is encumbent on them to return the change. This stops the lender being able to impose a delay - the 40 days will start from the day they receive the full fee.(posted 8615 days ago)Also send the #10 as a postal order NOT a cheque! Otherwise it's like saying here's the bank I'm with and heres my account number!
They already hold far too much info on you, why give them more.
Good luck.