Any experience/recommendations for GPS systems?

I'd say you'd pick up a few second hand "gypsy technology" ones at the markets as well.

Well the lads selling on the "gypsy techynology" at markets usually "borrow" them from the owners and sell them without asking 😉

I've done some searching and found some apps which are relatively cheap to purchase. You can connect them to consumer GPS receiver which give better precision than a phone GPS.

I reckon that I could get it up and running for under 300 quid. That is buying the GPS receiver, the app, and a new android tablet.

Don't have them yet but Garmin GLO 2 is one I saw. There seem to be a few apps that have free versions on google store. The most popular one appears to be one called field navigator. Again, I can't vouch for it

No. I think there are two technologies. If you want to go with the RTK ones you might need a base station. Those can get you down to 1cm of accuracy it seems. The GPS one says 3m accuracy but the apps say that using that GPS gets to 1 foot accuracy. Don't ask me how they can claim that

If you drive around it it probably does. I never tried that

You can add the fields into it manually as well I think. I can't remember exactly how to do it but I did it that way when I looked at it.