Quick update, I was getting the common issue with my belt grinder where the belt would track side to side with a decent amount of pressure applied to the platen. I lean on it a fair bit so decided to sort it out. For anyone else with this issue it may be a slight alignment problem but more likely to be a tension issue. My gas strut is 200N, I did actually plan to put two on but the first failed so I ended up with only one. Fortunately I had a packed up parkside pillar drill with a big spring in so I crappily welded the spring in as shown. It is TIGHT now and tracks straight and true no matter the pressure applied.
I also trimmed the platen down to the same width of belt and spaced it out a bit so the belt can hang off either side and I can get decent plunge lines on the knives both sides
Belatedly following @wyn 's advice and being continually hit in the chest by sparks I have fitted a few guards today. I had some 1mm mild sat around from an old project so used that. I also fitted one at the rear to prevent sparks and dust getting back into the motor and onto the workbench.