Need to setup a new method of payment

That's ok. 420s seem to be the same as here. You'll need the service key then.
Once you've made one, you'll find there's one key position past pgm2. That's the srv (service) position. To put a second cash button on, turn to srv, then type in:

951 - .(decimal point) - X - 45 - (press the position you want the button) - TL

The - is for spacing, so just ignore it. This will put the CASH2 button on and will include it on your end of day. To change the text to Gift Vouchers, go to pgm2 and type in:

2314 - .(decimal point) - X - 53 - X - (type in your text) - ST - TL

Thanks so much for this information... problem is I'm not seeing a couple of the keys you've specified, and whenever I hit the X key I get a Misoperation.

I don't see keys TL or ST... or are you abbreviating Total and SubTotal?

Yes I am. ST subtotal, TL total. Sorry, I'm that used to speaking the Sharp way when it comes to tills!

Actually... I've done a little digging myself, and found that the keyboards are reversed in the UK compared to here

Where porty has put an X use the @/For key and this should work