In the shopping centre car park. £15 for 24 hrs. Best you'll do unfortunately.
For convenience you can park in Union Square car park which I believe costs £16 per night. Cheaper option and only a short walk is College Street car park which is £6 per night. Both can be reached without the need to go outside, Union Square is the shopping complex that the hotel is attached to. College St. can be reached through the railway station which is also attached to Union Square.
Get an answer from Jurys Inn staff and past guests.