I haven't used them personally but others on this forum have and they too get a good review.
Then if you need 500, 550 or 600mm runners there's always screwfix. If there's an outlet near you then that can make alot of sense. You can also be sneaky. If there's a screwfix outlet and a B&Q Wearhouse within 10 miles of eachother, B&Q will match Screwfix's price and deduct 10%. Mad as they have the same owner. A 600mm runner from B&Q is something rediculous like £12. They are £8 ish from Screwfix so you could get them for £7.20. Still more expensive than Buller but you save the p&p.