Found 5th May 2017
Cheapest digital price yet at £39.99 and the cheapest available price on the internet, if not ever. £10 off it's original price (£49.99). It's on sale this weekend (until the 8th).
Use CDKeys to drop the price down to £35.12 by purchasing one of their £40 top-up cards with the Facebook code.
An even better 'deal' if you guys game share, £17.50 each.