Found this on Quidco yesterday, its the cheapest price I could find these type of bars at. Each pack contains 12 bars (not 20), and delivery is free.
If you have a Quidco account (why wouldn't you?) you earn 12% cashback on this purchase (£3.12).
If its your first ever order with Maximuscle use the code 'max09' to get £5 off any order over £25 (they tell you this in your confirmation email).
Not sure whether using the code will affect your Quidco cashback, but if it doesnt you'd end up paying £17.87 for 40 bars.
It is 12 bars per pack not the stated 20. So 24 bars total
It says on quidco link that: 'Cashback will not be paid if voucher codes are used.'