That would depend on two factors. Number one being if they're busy, if there is going to be another service then they're going to want you to move back to your seat fairly sharpish. Number two is if you've been a good customer. I'm informed that customers who were nice with the staff would often not be hurried back to standard. Passengers who were being obnoxious on the other hand...
Indeed. Be nice and the Pullman staff will work around you and not look to kick you back to steerage.
Engage with the Pullman staff, be friendly, chatty and proffer a decent tip and then they are in no hurry to bump you back to Standard. Get sh*tfaced, be obnoxious and start singing 'Happy Birthday' loudly to no one in particular and you'll soon find yourself being asked to settle up and move back to Standard before you've even passed Westbury.*
And based on my experience yesterday on the 1201 ex Plymouth, if you approach the Pullman crew at Plymouth before the set comes in, introduce yourself and chat about your forthcoming three course meal, the senior steward will tell you to take a seat in Coach G (1st Class) until they are ready to receive you in the Restaurant coach. That's what happened yesterday despite the fact that I and my travelling companion were holding a combination of Standard Class split (NRCoC 19(c)) tickets....
I did promise some more pictures of the scran. I'll embed them in the next day or two.
Also the rumour about additional Pullmans now seems pretty much certain from the May timetable change. Staff diagrams are being worked on at the moment and the HST sets used already have the full kitchen, being currently Travelling Chef services. The up will be the
Golden Hind, 0505 off Penzance with two breakfast sittings - one from Plymouth and one from Taunton. The down will be the
Royal Duchy, 1206 off Paddington, offering the same menu as the current Pullmans.
I will stress however that this info has only come from the current Pullman team. There has been no official corroboration from FGW. Watch this space...
Going back the OP's original query. Chatting to the Pullman team yesterday I was told that the down services off Paddington at 1803 and 1903 will almost always have space for Standard Class ticket holders on Mondays and Tuesdays. Wednesdays and Fridays are less likely but possible. Thursday is the one day in the week that you may be out of luck if you aren't a First Class ticket holder.
Finally, I'll just say, if you want these excellent full service restaurants to continue then there is but one simple mantra: Use it or lose it.
*Yes, I witnessed exactly that happen a few weeks ago on an up Pullman.