Make sure you have a new stat, and stat bridge pipe plus bridge pipe to rad hose...plus associated seals....if its the latest design of stat it needs slightly longer bolts and the stat bolts need sealent as they go into a water jacket...
Was thinking about this on the way home. Both times we stopped on the way home, the top rad. hose and bleed pipe through the TB to the expansion tank were stone cold. Massaging the top hose a little soon got hot water running round the system. Max also said that it was OK at speed but temperature crept up when stopped.
I'm thinking that a leak on radiator side of the 'stat would draw a load of air into the system in the 'stat closed condition because the coolant pump is sucking on the bottom radiator hose against a closed thermostat - hence (relatively) low pressure in this part of the system. Air getting in the top of the system seemed to be far more of an issue when underway than coolant loss. I think we topped it up once but it certainly wasn't losing a lot of water. It just needed a massage every so often to get the coolant flowing again.
The gas could have come from a HGF, of course, but there was very little pressurisation of the system so thermostat and transfer pipe looks worth a try.
Kevin