After several weeks of ignoring any issues with the Mini, I have pulled my head out the sand and have started addressing a few wear issues it has.

I am up to the front rubber mounts on the front subframe. Bugger.

Atm my mounts both haps gaps in between the subframe and the rubber, and the rubber and the body. Fairly sure they aren't suppsed to.

What is the easiest was to replace these (Other than have a mechanic to do it.) ?

The rubber mounts can all be changed with the subframe still in the car, as long as you do only one at a time. I'm assuming that by front mounts you're referring to the teardrop shaped ones. Once the two bolts have been undone then you can rotate it down and remove it. When the new one goes in, if there is a gap then install some shim plates.
Its common for new cars to have up to three shims. Its a square plate with a slot in the bottom.

Will have the car jacked up via the subframe affect undoing the mount, or can it be done with the car on all four tyres on a hoist ?

I've done it without a hoist, and without jacking the car up. A jack might be needed to help re-allign the holes at the end to get the bolts in.

Going to do it on a hoist to save my back