The key is to use dripping.
The fat needs to hotter than hellfire.
I take the meat out to rest then turn the oven up full blast with the pudding tray and dripping (goose fat/lard also works but not as tasty but FFS don't use olive oil it burns at the temperature you need. If you are accommodating vegetarians use sunflower or other high temperature oils).
When I take the tray out I put it on the hob with the gas on to keep it hot and smoking.
Quick as you can get the mix into the tray (I usually decant mine into a jug so it's easy to pour).
Whack in the oven and DO NOT open the door. You need to let them rise and "set" for at least 30 mins otherwise they will deflate.
If you're doing roast potatoes you need to time carefully as once those YP's go in you've lost your chance to turn/baste the roasties, so make sure you do this before they go in.
As for a batter recipe I can't help - I just make mine by eye like my DM did. For 12 YP's I start with 4 eggs, plain flour and milk and blend till it looks right - about the consistency between single and double cream. The only concession I've make is that I whizz mine up in a nutibullet rather than hand whisking 