I saw this whilst I was browsing my local B&Q (smaller one in Bury). I can't see it on-line and it could well be depend on which stores sell the product, so may not apply to every store.
Worth a look if you are in B&Q, looking to take advantage of their current 15% off everything voucher.
This normally sells for a fair bit more as I paid somewhere around £10 for some last year.
It's in the clearance section for £1 for a litre bottle, which is enough to treat the vast majority of heating systems to stop them rusting and damaging your boiler. A must for any central heating system.
With the 15% of voucher you can download from the B&Q website (valid till the weekend) it makes it just 85p a bottle. Bargain. I stocked up, so I have enought to last me for the next decade or more.
My local store had 40+ bottles of it.