Having picked up 2 Philips Senseo coffee machines for self/folks at Xmas from Argos/Comet from two HUKDs here... (thanks again to the OPs of those HUKDs!)
...have been looking around for a cost-effective source of 100g of Citric Acid to descale each of them with (as 2.5 months usage already) as insufficient descaling has apparently been a specific/historic issue with the machines...
(I was looking hard because the "official" Philips packaging of 3 x 50g sachets ie Senseo HD7006 is around £10, ie £6.66 for a 2-3monthly descale = the price of machine per year!)
I'd read that a chemist was a good source, but my local chemist plays very hard to get - ordering in specially to order in just 50g quantities at c£1.20 - because (from Wiki'ing) citric acid is mis-used for nefarious purposes. You can find it in Home-brew stores apparently, but none near me & postage makes eBay/Amazon sources too expensive.
This evening chanced upon the fact that John Lewis sell a well known brand in 250g pack size for just £2 (with free-delivery to store). Two packs s/be enough to see your machine through a year.
Here's the Philips link to "how to" http://www.misc.philips.com/senseo-descaling/us/en/descalingHD7805HD7826HD7840HD7841.html
(Note that despite the mention of 50g at step 1, you find you need another 50g at step 7.)
Hope this deal (better than any other I've seen) helps you keep your Senseo coffee makers (and similar) going strong - eg with its Lidl pods (c£1.50/18) and/or from CoffeePodShop.co.uk (Netherlands) - we all love our machines!
JL will ship - but want £3 for their Royal Mail service.