Im no computer wiz myself but I use the eVAT system to file all of my clients VAT returns. I found the process simple and very user friendly, with the added advantage that my clients can never lose their VAT returns again!
You may also want to get your client to pay the VAT bill by Direct Debit, as they will receive 7 extra days from the standard due date to pay. This extension can create a substantial cash flow benefit to your clients - so this should keep you in their good books, it has for me!