I assume that this would be all on one ticket? Or at least hand baggage only?
If you are transferring directly between T4 and T5 airside then there is a direct bus. Check out this:

http://www.heathrowairport.com/heath...-international

However, if you go outside to have a cigarette (no indoor smoking in the UK), then you will have to go through immigration, then find your own way to T5, which isn't very quick (either tube or HEX via T123, or take the normal red bus 490 or 482 (for free)). You will then have to be entering security in T5 at a minimum 35 mins before your next take off.

Personally, I think you'll be very pushed for time if you do that. However, if you transfer using the airside bus, you should have time for both champagne, and a trip to Boots for some Nicorette

(Self Gloss, my 1000th post!)