A little late I know but for anyone who comes across this problem try running OnyX (if your on OSX). Its solved loads of niggly problems for me in Flash, Dreamweaver, InDesign and Photoshop.
You could ask them for a Purchase Order? I've sometimes sent a copy of the brief with an estimate attached to the client and requested they sign off both. I'm sure there is a more sophisticated name for that process.
Someone once said to me that setting out your stall from the get go and voicing concerns as soon as they appear will save heartache and lawyers at the end.
VAT is a headache, and I'm not VAT registered but have worked very closely with persons who are. As your not VAT registered I'm sure (I'm no accountant) That you can't add VAT to your invoices and you can't claim VAT back.
Taking all the above into account, I think your about right. If I was in the same position I would add everything together:
PRINTERS BILL £100
PRINTERS BILL WITH VAT £117.50
YOUR BILL £50 TOTAL £167.50
