Okay, leak now sorted. For anyone else with this my assumption that the rear of the car was dropping both sides evenly was VERY wrong and after advice to actually measure what was happening (I measured each side every hour for a day and part of the next day) I realised that the RHS was dropping whilst the LHS was static until it had to drop to keep even (there's a levelling mechanism apparently).
The whole repair procedure is a real pain though as the rear seats and parcel shelf have to be removed to get to the top of the strut. On removing the strut my garage found that the shock absorber had been leaking causing a bad seal with the strut and that seems to have been the root of the issue. They managed to source new aftermarket parts but even so this is not a cheap repair: parts were about £1,000 (I did NOT know a shock absorber could cost £600) and the procedure required a lot of labour to carefully remove and then reinstall the parcel shelf and seats.
All done now and completely stable after 48 hours.
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