It's got 700c wheels and at the moment I'm running 35mm Schwalbe Marathon+

Can I put some really narrow roadie type tyres on it? And do I have to get tidgier inner tubes if I do?

Sorry for sounding absolutely clueless here…..but I am.

Your advice is warmly welcomed.

should be able to – just be wary of the rim size.

there is s a (red and green) table here http://www.sheldonbrown.com/tire-sizing.html (measure rim inside across the bead hooks)

yes you probably will need smaller tubes.

Can I put some really narrow roadie type tyres on it?

Yep. I wouldn't go any smaller than 25mm though.

And do I have to get tidgier inner tubes if I do?

Probably. The size range will be imprinted on the tube somewhere.

Unless you want the weight savings, just pump up the tyres to 100PSI and see if that improves matters. Personally I don't go narrower than 28 on anything other than my road bike and they are 25. If you go that narrow you'll need new tubes too, you may need to switch valve type to presta.

bike will feel a lot faster – due to loss of weight and decent carcass if you buy a decent tyre.

Otherwise what they said.

or try some 32c conti sport contacts or schwhalbe kojaks – still fat but quicker – marathon pluses are heavy and slow (but bomb proof!)

as djaustin says i have 28 on my commuter bike, best compromise