Thanks for the help guys, I'd assumed that leather would be more breathable with it being a natural material, it's good to have advice of people who have tried both rather than trying to figure out what is best from adverisments.
How do man-made materials compare to leather in the event of a slide down the tarmac, has nylon etc caught up with leather's protective properties, are there any materials that I should definitely avoid?
Once again, thanks for your help.

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Never do today what can be put off 'til tomorrow

Ha, they've moved far beyond nylon, think more about kevlar and all sorts of other sturdy materials that you've never heard of. Not sure if I've seen any scientific studies but I've certainly seen lots of glowing reports about how well many of these jackets do in a crash to convince me that they are good enough.

You need to work out a system that works for you. There will be layers. There will be at least one base layers who's function is to control heat. Then there is an outer layer whose function is protection from the wind and any off. Finally there might be a waterproofing layer to deal with serious downpours and give you something that can stay outside the tent when soaked. Materials:

Cotton (basic): cheap and cool.
Wool/wool pile: warm ,cheapish, works when wet.
Goretex: breathable and showerproof when in first class condition and clean, like a Tesco's bag when blocked/dirty/old.
Leather: Tough, windproof, will work after a crash, useless when wet.
Waxed cotton: totally waterproof and windproof when treated, leaks when it isn't. Semi-crashproof if top quality, breathable.
Nylon: waterproof or breathable but not both. A swine to get fasteners to stay stuck to. Burns/melts at the touch of the road or an exhaust.
Aramid/kevlar: very strong in tension but expensive, lacking any other useful feature.

Modern bike gear is basically a nylon coat with Kevlar strands for abrasion resistance, foam rubber armour to give impact protection and a Goretex liner (inside the nylon) to make them splash proof. Clean them every few trips to keep the pores open and/or use a closed oversuit to stop the water overwhelming them and they work. When dry and clean they breath. They are designed for weekend riders, so if you use them expect to pay out £400 every two years when the press studs drop off and the plastic zips fail, or £800 for something that might last a bit longer. Fall off and you go shopping.

My own preference for a trip where I'll ride all day (I use modern stuff to sling on for a trip to the shops) is using wool pile and silk under leather trousers/armoured jeans for toughness, a treated wax cotton coat for temperature control and water resistance with nylon over trousers as required. I have not solved the armour issue for solo riders but I feel this combo has worked out cheaper and more practical over the last couple of years than the commercial Goretex stuff I used up to then. The Kevlar and cotton jeans you can wear to the pub too, old styles of wax cotton kit have far superior collar and cuff designs.

Leather is still the choice of racers, it will not be beaten for abrasion resistance and when armoured with hard plastic does the job. It is a one trick pony though, you fry or freeze unless it's that one nice day in May. A one piece Orange and green suit makes you look a right *** if you are fat and fifty .

You need to pick what works for you, not just go down the shops and buy what they sell.

Andy