[Casey76]

I noticed the other day that one of my local garden centres had autumn (well July-Oct) planting strawberries.

Is it normal to plant strawberry plants at this time of year?

If I was to get 24 plants, is it better to get a mix, say 6x3 varietis, or just stick with the one?

How much space would 24 strawberry plants need?

When do you need to mulch around the plants

(so many questions, I've never attempted to grow strawberries before )

[Trillium]

Not really heard of such late plants myself but so many new types are becoming available. If they're selling the plants now, then get them planted and you'll likely get a good crop by next year.
Varieties: you can stick with one, perhaps a 2nd and see which you like better tastewise and as a producer.
I space my new plants about 8" apart as they  throw off runners later and need the space. Mulch can be applied anytime to keep the roots moist and weeds under control. Just remember that the plants must be in the ground at the same level/spot they grew previously, not higher, not lower. Its a fiddle but necessary.