Sorry for the long delay in responding back DanielW.
I think I may have accidentally found the solution today though.
The space on my laptop was running low for no good reason. I eventually narrowed it down to the UE4 derived cache data in my user profile. There was 30gb or so of stuff sitting in there. I did some googling and found out it was OK to delete all of this as it gets regenerated as required.
Since then lighting builds on my laptop no longer seem to have this issue. 