The Portsmouth CTC web site has two fascinating articles explaining how the Billy Trail came to be developed as a shared cycle path.
I hadn’t realised the origins of the CTC Havant rides were so closely linked to the Billy Trail. See:
Footpath 101 closed for up to a year
Footpath 101 runs behind Mengham Junior School. The closure starts 28th February 2015.
You can see more detail including a map and suggested alternative route at: