Is this not a done thing? I put a load in peat pots to make it easier to
plant them out and they haven't liked it at all. They have barely grown
compared to the others in plastic pots.

I've planted all my tomato plants in peat pots. They are now nearly 2 foot
high, very thick stems, and throwing out flowers all over the place.
However, there appears to be a trick to using peat pots.

You've got to keep the roots damp. I made sure of this by cramming the peat
pots together in a seed tray. There was no gaps between them at all.
Although they were round, they got 'squashed' into a square shape to fill
the gaps. This kept the peat pots nice and damp so when the roots came out,
they were still happy. When I potted them on, I soaked the peat pots
thoroughly first, then carefully tore each pot open and folding back about a
quarter of the pot. This meant that if the pot dried out, the roots could
still exit into the new compost thru the hole.

Did the same for my squash, cucs, pumpkins, courgettes, beans, datura,
incarvilla, peppers, etc etc

Best,

- h