Over the last few days it has taken longer to get to temperature but when it has the correct temp is maintained. I've ordered a new thermostat but feel the temp would remain below normal if it had failed.

If its failed partially open eg some debris in it, that would give you slow warm up but normal temperature at other times. BMW i3 Electric Car
2012 Full Fat RR 4.4 TDV8 (now gone)
2006 VW Touareg 3.0 TDi V6

Glibbs- two questions - did this fix it?

More importantly did you have to buy the whole house if for £400+ or did you manage to get just the stat itself? Current stable:
2006 BMW M6
2007 4.2 V8 S/C FF
Gone:
2002 4.4 V8 FF

I found a rubber gasket which was part of the thermostat itself had somehow become dislodged. I purchased a new thermostat prior to start work so I installed that and all was well apart from the fact that a housing seal was not included with the new thermostat which resulted in a small leak (a cupful a week) until yesterday when I installed a new one. Caution - the lower bolt of the housing is only too easy to drop never to see the light of day again!

Thanks guessing you didn’t remove the whole housing? Just the thermostat end with three bolts so you can access it?

Indeed.