Apologies, I didn't realise quite how long I'd been hanging on to this... I'm only an occasional cacher these days, so I did a "TB audit" recently to try to identify suitable caches in which to drop off the various TBs that I've picked up over the years.
I'm not sure why I didn't think of Windsor Castle a long time ago, as it's an easy enough drive from home. Perhaps I did, but there were no obvious caches - nothing close to the castle that was big enough - in which to drop it off. Any which way, there were still no obvious ones when I looked again as part of my "TB audit". So, I had to compromise slightly - and pick a cache from which a castle is visible!