We have just installed 3rd gen Nest on S Plan system (using L, N, 3, 6 and linking L-2 and 2-5). Central heating and hot water seem to be working. Issue is boiler is running status 'c' constantly rather than going to '0' standby mode - as if there is a constant demand for heat? There is a constant noise too which sounds like it may be a fan? Any ideas what this might be - wiring issue or boiler problem?