I thought I would update you on my progress.
I have successfully integrated sagepay and am running through the testing of the system.
It successfully gives the details to the simulation servers and handles redirects from the simulation server to the receipt page or a custom error page.
Sales pages are updated as expected.
There is an added section in settings for activating sagePay.
It works for the form version of sagePay simulation. A real account can take up to six weeks to activate, but will update to see if it successfully works with this.
By the way I was having an issue with sending Emails, I found this was because they were being blocked from reaching the inbox by google and hotmail. I found that if you use a plugin that changes the wpmail and sends it via smtp this problem is overcome. WP-Mail-SMTP
Also, when the customer is looking at larger versions of the photos, where the background fades dark, I found that the user could right click and download the image (which I didnt want). This was overcome using a right click disable plugin No Right Click Images Plugin
I have had to change a small bit of your code obviously adding to settings and adding custom pages.
Any larger bits of code that I needed I copied into a new function to ensure other payment gateways were not affected.
I can send you the code, but this plugin has generally all the hooks and actions that you need in the right place.
I have been only using wordpress for two weeks and have got this far. I only had a limited knowledge of php before.
Thank you for your help.
Leon