hers - slk 320, his - dull diesel BMW fleet :/, Xtrail, Honda CBR 1100xx, Yamaha YZF600, Ribble Road
Peter - if you just want to treat them then Autoglym's leather care set is very good. It's a two part treatment. Similarly - any of the good names, Meguiars, Swissol, Zymol etc
Gliptone Liquid Leather works really well. Treatment in two stages:
1. Liquid Leather Gentle Cleaner - Safely cleans leather products without damaging important stitching. Cleans quickly and thoroughly prior to conditioning without the use of water.
2. Liquid Leather Conditioner - Restores, softens, protects and really does reinstate the smell of finest quality leather. Suitable for car and domestic seating, cases and clothing, etc.
Bought mine from eBay - Just over £15 all in for the two bottles.
EDIT: Check prices before buying these from eBay - Price is cheaper direct from their website and delivery is inclusive too. £12.30 for the two bottles delivered. You live and learn
I am a little dubious about its "deep down" effectiveness but it does have a great smell.
I leave a cloth with some of the conditioner on it in my car to really impregnate the smell into the car's interior.