Thanks for the tips. The multi-story worked fine and, being a quiet afternoon many spots available. I walked down to Ground as was not sure where the walkway connected? As my visitor was badly delayed in passport control (on account of few humans being on duty, gosh what a surprise!) from landing to exit - with NO checked bag - took an hour.
The interior of the car park, stair wells etc was down to the usual LTN standard of don't pay anything you don't have to. I was in the car park for 1 hour and 16 minutes, so £18 puts them in LHR territory and they are clearly aiming to make their money back very quickly.