Just thought I'd update you all, after looking further in to other products available thanks to the advice of you guys here, I decided to go with SliQ anyway. Everything else was either online based or too much for what I need.
I currently use an excel spreadsheet I built myself for my accounting and it works perfectly, I only needed invoicing software and SliQ does exactly what I want. Other accounting options would at this time mean paying for more than I need.
SliQ was only 50 notes and updates are free.
If anyone wants to know more about it then I'm happy to explain. Having now got to grips with its basics, it has already sped up the invoicing process ten fold. Input the customers details and numbers and with two clicks it automatically sets up a customer account, emails them the invoice and then tracks it to see if paid or unpaid. If it is approaching overdue status it automatically emails a reminder to the customer, then once again every seven days past overdue. If payment is made then upon clicking the 'paid' button it automatically emails a receipt.
Upon partial payment receipts are automatically emailed and track payment dates. Paid invoices parse the amount over to my spreadsheet. Best of all, invoice templates are completely customisable and invoice numbers are automatically rolling. VAT is automatically tracked so upon each quarter I know what I'm paying without having to do a single calculation. Suppliers invoices are tracked, as are costs.
It is quite literally exactly what I was looking for and no more.
Thanks again for the suggestions.