Unable to start Hyper-V VM “Not enough memory in the system” error

I found I was getting this in my test lab even though there was enough RAM spare on the host (approx 75% utilised only) turns out that Hyper-V is reserving a fair chunk of RAM automatically for the operating system (Server 2012 R2 in my case) therefore I just needed to lower the minimum reserve value.

Regedit РHKLM / Software / Microsoft / Windows NT / CurrentVersion / Virtualization

Create new DWORD “MemoryReserve” with value Decimal 512 to ensure minimum 512MB always remains after VMs are started.