I have a Shimano LX Chainset and possibly need to replace the outer and middle rings. Am I better sticking with Shimano or going to an alternative. I have seen some Deore rings on Chain Reaction website that are much cheaper than LX/XT replacements. Is there much difference?

Most cheap chainrings are stamped out of (fairly weak) steel. This means they tend to wear out (ie their shifting performance degrades) more quickly. More expensive ones are CNC manchined, as this produces a better finish, and the material is too stiff to stamp anyway. If you have an LX chainset buying Deore rings would seem a bit of a false economy, they'll last less time than your current ones. I've always used BlackSpire replacement rings and never had any trouble.

I always assumed that steel rings would last longer than aluminium as steel is stronger, is that not right ?

Strength and hardness are different things. Diamond for example is ridiculously hard but very brittle - you couldn't make bike frames out of it... 7075 aluminium alloy is pretty hard, and the steel that pressed chainrings are made out of is cheapo crap.

Are Truvativ ones any good?

I'd say either go for cheapo ones (Deore or Truvativ steel jobs - e.g. £3.99 for a steel middle ring!) or pretty pricey ones like Middleburn Hardcoats.

Chainrings like LX & XT ones just don't seem to last.