Thanks for your enquiry.
I am afraid that as there is over £5,000 in the ISA, you will need to obtain Probate.
For the sake of a phone call, you can always phone the Bank and explain that this is the only asset, and see what they say,but I'm afraid all major Banks will insist on seeing Probate if the balance on their account is over £10,000.
Please don't worry too much about completing the Probate application- they won't fine you!! If it is getting you down, you can always ask a local Probate Solicitor to sort out the paperwork for you- he can prepare an Oath, which you will need to swear, and he can then submit the Oath and the Will to the Probate Registry. He is likely to charge approx £150-£250 plus VAT for the work.
I hope this assists and sets out the legal position.
Please let me know if I can be of any further more assistance.