Restrict Banking Details in iSupplier Portal

Good afternoon gurus, We have started using iSupplier with the bulk of our Supplier base in April.

We are an operation solely based in the UK

At the moment our supplier can input just about what they want into a field called Sort Code

We would like to restrict this to

6 characters  ~ all numeric

Trouble is, have never used Personalize Page  before ~ scary

I would have guessed this would be quite easy but the options do appear to be very scary.

I have put these in the attached

Can anyone (presumably in the UK) help me execute this?

I would be externally grateful