@Tristram Shandy (not PRIME 20!)
I was the Technical Support Manager for the DECSYSTEM-20s at Wilton from 1984 to 1990. You are referring to the PMS system running on HQ2. This application was an early attempt to move up a level from SCADA and other Plant Control layer systems to provide management information at a higher layer. PMS formed the foundation for a later application known initially as PMC and later as 'QUESTAR' which was implemented on VMS and widely used across the ICI group in the later 1980s and 1990s.
PMS took readings (over RS232 async terminal type ports) from some control systems but primarily from VTx00 terminals from plant operators. It was only ever implemented on the Wilton Nylon Inters and North Tees Aromatics plants.
ICI ran three DECSYSTEM-2060 systems at Wilton and a further one in the NETCC at Billingham - there were also a couple of DECSYSTEM-2020s which were more or less inactive by the time I arrived. Before ICI I worked for an East Midlands Polytechnic supporting their 2060 so I do not know where your suggestion that there was only one other in the UK came from. The population (at peak) was at least 30 and they were widespread in the Higher Education sector, including (IIRC) :
Polytechnic of South Wales
Robert Gordon's Institute of Technology, Aberdeen
Glasgow College of Technology
Bournemouth Institute of Technology (? name)
The Open University (3 sites)
and that is not taking into account the many other organisations that ran DECsystem-10s (including ICI!)
The architecture of the PDP-10 family (including all models of -10 and -20) was indeed 36-bit an d implemented a variable byte length of anywhere between 1 and 36 bits, depending on the byte descriptor.