There's currently a Subscribe and Save offer at Amazon on Finish products (as well as products from other brands including Vanish, Dettol, Airwick and Megared).
Using code "SAVE30PC" you can save 30% off the subscribe and save price for the next delivery of the selected items.
There's a choice between the Powerball packs which are advertised at £10 for 78, but with a 30% discount are £7.00 (9p per tablet):http://www.amazon.co.uk/Finish-Lemon-Sparkle-Dishwasher-Tablets/dp/B00820VLNU/ref=sr_1_1?s=grocery&ie=UTF8&qid=1404824244&sr=1-1&keywords=finish+dishwasher+tablets
Alternatively there's a 45 pack of Quantum (I went for these) for £7.72, which works out as £5.40 after the discount (12p per tablet):http://www.amazon.co.uk/gp/product/B00GUEV612/ref=noref?ie=UTF8&psc=1&s=grocery
This item must be purchased directly from Amazon.co.uk to qualify for the Save 30% with Subscribe & Save offer
Save 30% on products from brands including Finish, Vanish, Dettol, Airwick and Megared. Simply enter code "SAVE30PC" at check-out to get a 30% discount on your first subscription to eligible items. No commitments or fees, cancel any time. Offer only applies to Qualifying Items bought and dispatched from Amazon.co.uk. It does not apply to purchases made from third-party sellers at Amazon.co.uk’s Marketplace platform. Please select "Amazon" in More Buying Choices on the right of this page, and add to basket if you still wish to take advantage of this offer. Learn more.