Answering my own problem here. It seems that Sketchucation tools is the culprit. I transferred the version I was using with SU 2016 and used it alongside the plugins that the Trimble extension manager had installed. Having removed the SU Tools extension, memory requirements dropped dramatically to something reasonable.
Doing a bit of research, I found SU tools v. 3.1.3 but could not get them to install in SU2017 Pro 17.3.116 - I unzipped the rbz and put them in the plugins folder then relaunched SO but to no avail.
It seems that Trimble are determined that one should use their extensions manager.