On your purchase entry (bill/cheque) all you should have to do is entry you customers name/job under the 'Customer: Job' column and then put a check mark in the box under the billable column. If the company preferences are set correctly, the prompt to assigned billable expense, or amounts should automatically pop-up when you go to create an invoice for the client.
Though it looks like you will have to over-write and add on the VAT paid on the purchase to the amount charged to the client on the invoices you create. The alternative would be to forgo the alternate tax set-up and just enter the reimbursable purchase items you enter on bills/cheques as including the amount of VAT and as a non-taxable item. This way when you go to create a customer invoice the total amount will be entered, and you will not have to go back and double check the initial purchase, and VAT charged.
