To continue from before - unfortunately the battery wasn't the only problem with the Skyline. Once the battery shorted, it took out another part - known as an Ignition Transistor. Sounds simple enough, but it costs a whopping £492 from Nissan prior to adding the dreaded VAT on top.
Factor in the cost of labour and the costs already incurred and I was looking at over £800 in repair!
Fortunately, the mechanic gave me the part number and the option to try and source the part for myself, albeit cheaper.
First point of call was a local garage that services import Japanese cars. However, they quoted me exactly the same cost for the part - as it's only available from Nissan themselves.
I finally opted for a trawl of the Skyline Owners Club forums hoping to source the part. After a few minutes, I had exchanged several pm's and had the part being sent first class the next morning. Perfect. All for the much more affordable sum of £39.
This morning the Ignition Transistor arrived, so I went home at lunchtime to take it to Nissan. They're fitting it and getting back to me tomorrow - to let me know if everything has been repaired successfully.
So, things are looking a bit brighter, but I've still got my fingers and toes crossed and am constantly touching wood.