Whats the best 'all in one' feed. tried and tested? would it work out cheaper?

It doesn't necessarily work out "cheaper" to feed the all in one horse feeds. As horses are intended to graze for most of the day, a good quality gr hay is the best feed. Give them access to minerals and salt to encourage them to drink and monitor their weight. Hay will help them keep warm as it's digested slowly and helps them maintain body temperature. If they have plenty of hay and drop weight, add some kind of concentrated "fattening" feed. Your custom blend is probably fine.