Just thought I'd add to this guys,
This morning I bought the replacement cable from BMW to repair the bonnet release. The cable was £19.03.
Mine snapped at the handle inside the car so luckily it only needed the first part of the cable Part No
51238402614 (Item 10 Shown Below)
I called a few local garages and was quoted silly prices to fit it starting at £150+ and they couldn't even tell me for sure how much it would be since they would first have to get the bonnet open and they didn't know how to actually do it so It could be a huge job. I was just told "how long is a piece of string?..."
In the end I took it to BMEuro (a BMW Indy in Wolverhampton) who said they knew how to get the bonnet open and had made a 'special tool' to get the cable through the bulkhead. They quoted £40 for the work.
It took around an hour and now I can open my bonnet again!

It would have been a much bigger job if the other end of the cable had failed meaning that all the latches etc would then have to be changed.
From now on I'm going to be keeping my catches and cable throroghly lubed up!
Luckily I was only lifting the bonnet to clean the engine bay on my drive when my cable snapped, but I would have been well and truly F'd if I had broken down at the roadside and the cable snapped there, or I needed to top up with oil etc! I don't know what I'd have done.
Looking at other threads on the subject they all say that the only way into the bonnet is to "smash" through one of the grills, then break some of the plastic trim behind them to get access to the cable to release the bonnet....Glad I don't go for that option!