Thanks for the link to justfans.co.uk Gareth, they were really helpful as you said. They had the specifications of the components to hand and did a sanity check on my figures. All in order thankfully. I was able to get a suitable filter and various other components from there too.
I bought the "TD-SILENT 500/150" inline fan: https://www.justfans.co.uk/silent-500150-p-1906.html
As a bonus, they're recently upgraded those to three speed. When I first specified it last year it was only two. The third speed is slow, it should circulate enough air for several people to breathe while keeping as much heat in the room as possible. Useful during winter. When the fan was two speed only (medium and high) I would have had to buy an additional variac controller to get the slow speed. No need for that now, this simpler and cheaper three way control will do the jobhttps://www.justfans.co.uk/mts10-extrac ... -1171.html
If my calculations are correct, medium speed should be enough to keep 5 people working hard in the room cool. High will be super turbo chill