In reply to TobyA: You could do a Ghetto Tubeless conversion.
I recently did this on a Whyte 901 and rode the bike down a canal toepath last Sunday, renowned for Hawthorn.
I found 8 punctures, ie thorns well and truly through my tyres, not one flat.
just pull thorn out and it self seals.
So my point is, buy almost any lighter tires save a bit of weight and dont worry about punctures,
It may seem a bit of a faff but it works on almost any mtb rim and tyre combo.
Cons. you still have to carry spare tubes, to be sensible, you should not run your tires over 40 psi(recommended) unless an actual tubeless rated tire.
you will need to top sealant up occaisionally.
Pro's eliminates the puncture resistance(how long is a piece of string question,,,,sorry
), saves hours of stopping to fix punctures, allowing you to ride more,
Sealant can be purchased £20 for large bottle £2.99 each 20" BMX tube needed for conversion,