I'm trying to shortlist the best rated sub £250 (seemingly equivalent to
$400 worth of American speakers) active monitors before inflicting my
indecision on an unsuspecting music shop.

I've heard mostly good (and some bad) things about all of the following:

Alesis M1 MkII (£225)
Behringer 2031A (£220)
Event TR5 (£240)
Fostex PM0.5 (£200)
Samson Resolve 65A (£200)
Wharfedale Diamond Pro 8.2 Active (£250)

I have a small room (8.5' x 8' x 8') and am forced to wall mount the
monitors as I am sitting in front of a nice, big, reflective, fitted desk.
With the right brackets, I should be able to get them at least 6'' off the
wall.

I will be mixing mostly techno but also some acoustic guitar, percussion
etc.

The more power they've got in reserve the better.

I appreciate that what sounds good is very subjective so I suppose I'm
looking for advice on the technical merits of the monitors above and any
obvious contenders I have missed.

Cheers,

Marc