In light of the recent English Heritage gardener pay rate fiasco (see here) it's worth bringing up this post from 2008.
Bullingdon Prison were advertising for a groundsman / maintenance operative on a minimum of £20k even back then.
To throw into the fray - I was offered 18k (£18,300) + Cottage, all tools provided and fuel allowance of £10 per week to be a live-in gardener on a small country estate is North Yorkshire in the past month - I turned the job down as It would mean giving up my business - but to give a feel for the situation, employers who value quality work, ARE willing to pay for it, and ARE paying fair wages with a good benefits package. The English heritage job was a rude p***-take, not a joke! you can earn more, with a better benefits package with ALL major supermarkets, and without the need to get qualifications than be a gardener at that wage!