Thanks for the info guys, I'll look out for some decent 17cm units to fit in the doors, Golfpro - I'm assuming that you fitted the tweeters behind the dash grilles?? I'd prefer components fitted this way if possible as they are bound to give better performance than co-axial units.
The sub I'm hoping to use is a little, purpose made under seat unit, I think the driver is only about 5", fitted in a ported, plastic moulded enclosure, and it comes with it's own seperate 50W mono amp and adjustable low pass filter. OK, so I'm not going to get crisp megabass or serious SPL's, but I have used a similar unit previously in a Opel Manta with surprising results, nice amount of bass added at reasonable listening levels, with minimal disruption to the car's interior.
Still haven't got to the bottom of the poor reception problem yet though. Bazzer - I've checked the power supply connection and it stays live as long as the head unit is on, regardless of which source is being used, so it isn't that, thanks for the suggestion. Apparently the ariel amp is located in one of the rear pillars, behind the trim, so that's my next check. May take me a while as I don't have much spare time at present, but I'll post results when I get to the bottom of it!!