Will this (http://www.caraudiodirect.co.uk/dynamics-s2002-p-5528.html) amp fit under the passenger seat in my MY03 blobeye?

How's best to secure it down or is it's own weight enough?

I think it will - I have just fitted a Genesis Profile 4 under the passenger seat and there's loads of room - it is a little smaller than that but not much. Mine is a bug but I'm guessing they're similar.

It will need to be secured - ideally screwed down, but that is a bit impractical under the seat so I went for velcro which does the job.

Yours: 222 x 54 x 300 mm

Mine: 217 x 38 x 300 mm

Reckon you will have no trouble.

Cheers mate. Where did you get your velcro from?

Robert Dyas, or any car stereo shop will have it, maybe even halfords.