Had it topped up a couple of years ago & definitely working last summer but driving up to London the other day, it wasn't. Pretty sure it was working when I was off-roading back in January (it was constantly raining & often in/out car allowing water in). However, the car has not been used much since then.

Into KwikFit this afternoon, they hooked it up but after an hour (happily spent watching TdF going up Tourmalet) came in and told me there was a leak. They haven't the expertise to advise where the leak is but I also didn't get charged anything.

So, now I can ask the experts - where is this leak most likely to be?

FWIW I always have the A/C on auto.

Most likely the condenser. It's the big rad right at the front of the cooling pack, and very susceptible to damage from stone chips and road debris

The correct method of testing it to pressurise with Nitrogen and soapy bubbles

There should have been a dye put in it at qf so you can find the leak with a uv light.
As stated the most likely place is the lower edge of the condenser but it could also be the compressor seal.
In this instance I would say condenser. I had mine replaced last year for £402 (05 supercharged) all in.