So.....the function happened...and I have to say...you were all RIGHT!!!!Thanks for your replies.
I've done practice runs with these beef Wellingtons with a mushroom duxelle but only 10 max. I've prepped them myself (not a pre bought frozen product.) 20 min blast in a 200c oven and then down to 180c for 40 mins, until internal temp is at 45c which is medium rare. It's worked great. Puff is perfect and meat as well, but that was only with 10 BWs.
The bricks laid at the bottom of oven is helpful. I will let you know.