Hi chevron, I thought you might have some info !
The gantries at the NATS units (including Edinburgh, Stansted, Birmingham, Manchester and Heathrow) were all identical so it's a reasonable assumption that they were specifically designed for the Watchman. Whether that was a Plessey option or a particular NATS requirement, I don't know. Interestingly enough, the Watchman aerials at most of these units were stripy red and white. Just plain red at Leeds Bradford.
The shorter, slimmer gantries at the likes of Leeds Bradford and Southampton are identical among themselves, though with some height variation. When the LBA Watchman was installed in late 1989, it needed a completely new site as the ACR430, which it eventually completely replaced, remained in place on its concrete plinth for a few years. Perhaps a low concrete plinth wouldn't have been suitable for a Watchman anyway.