I work in insurance - you may be able to successfully argue a 50/50, but the onus is on the person performing the manouver to ensure the way is clear. You may be 100% liable for this dependent on what companies are dealing - gawd bless Norwich Union and thier Indian call centre workers who have no idea about british road traffic laws!!
If you're reversing, then you're not going with the normal flow of the traffic, and whist you may be able to argue contributory negligence on the third party for pulling up close behind you, you should be looking where you're reversing 100% of the manouver.
It doesn't appear to be fair, but if the third party is travelling in the correct direction in a carriageway, it's his right of way.