Prestashop Product Multibuy Offer Module

Project Description:
Using Prestashop (version 1.4.7.3) we require the ability to setup an offer so that if multiple products are bought from the same category then the product price changes.

We are currently able to setup products in Prestashop so that if a quantity of the same product is bought then a different price is applied using the Back Office > Catalog > Prices > Specific Prices, but want to develope this to be able to add rules so the specific price is applied if a given quantity of different products within the same specified category are ordered.

Example
10 different products in catagory A with varying prices £5.99 to £7.99
If any 2 of these products are ordered each product will be at the new price of £5 (any 2 for £10 offer)
If any 3 of these products are ordered each product will be at the new price of £4 (any 3 for £12 offer)
If any 5 of these products are ordered each product will be at the new price of £3 (any 5 for £15 offer)

15 different products in catagory B with varying prices £2.99 to £4.99
If any 4 of these products are ordered each product will be at the new price of £2 (any 4 for £12 offer)

Must be able to work with a number of different offers running at the same time.

Note - We are currently use the 'Maxi Promos' Prestashop module for these offers but this uses the voucher discount method, offering the price reduction as a discount for the order and does not alter the product prices which is our new requirement. It also only offers a set discount so all the products in the offer have to be the same price which is very restrictive. This is why we have this new project request for a new improved Multibuy Offer Module.