If your ISA account has an account number, sort code and is with a provider that has branches or can be managed by post,you should be able to deposit a cheque. The maximum you can invest in the 2012/13 tax year is £5640.
I would have though so. Who is the ISA with? You could check the terms and conditions online. If the cheque is made out in your name then they would need the account details - you could write them on the back of the cheque. If you are posting the cheque then the details could be on a covering letter too.