Back! Sounds like you're organised, OP, but I'll leave this here in case it's useful to someone else.
The van was tucked along the side of the hospital building, apparently it's usual space, which I have never noticed before. Very discreet.
Six to eight stairs up, along the side of the van, onto a wee porch with bell, speaker, locked door. A disembodied voice checks name and appt before letting you in. There's a wee office/reception cubby to the left and a padded bench opposite the door to wait. Two curtained changing cubicles along a corridor to the right, with a folded wheelchair opposite (possibly for use if personal one is too wide?). At the back, a closed room where the scans are actually done. Lots of space inside. It's set up so one person can be in appt/leaving and another arriving, and that's it really. It feels entirely private when you're in there. Toasty warm too. Two members of staff that I could see, one doing a screen with the previous person, one who welcomed me then did my screen. She was lovely, and very deft with the distracting chat during the screen itself!
I had a few minutes to wait, so I asked questions.
No toilet. Hence parking up at hospital/supermarket/place with facilities nearby.
There is a lift! The stairs flatten and convert into a shallow ramp, platform at top of stairs is actually the lift. My invitation letter said to call if xyz, one of which was using a wheelchair, so perhaps there is a choice of travelling to a physical centre or reserving a longer appointment at a mobile unit?
Final note: I'm in Scotland. We have six centres, in the big cities and south, but are well used to mobile provision for all sorts of things in the north. This van travels all over the Highlands and Islands. It's a big thank you NHS Scotland from me 💙