I live in mainland UK and I have some red bud seedlings 3 sprouted fron 2 seeds in a cell although 1 appears to be stunted and I have 1 pine all in cells. My question is when should I repot these and what should I move them into ?

Amazing they actually germinated. These seed kits are expensive and don't usually come to much. You will have to leave them seedlings until spring probably as they need to be big enough to handle easily without damaging them.

Trees germinating off season generally grow leggy and weak. Try to keep them cool and give them as much light as possible. If you have strong plants come spring, move them outdoors. First in full shade and slowly transfer to full sun(4-6 weeks). During the season uppot to bigger pots as it grows. This big leaf species needs a very thick trunk. Just let it grow freely for 5 years or so. In biggest possible pot or in ground. When apropriate cut back to a new leader. The longer you wait the better taper it gets. Then repeat the process a few times. When trunk thickness and taper is as you like it is time to transfer to bonsai pot. Generally it is best to start in an oversized training pot and do the transfer over a few years. Experienced artists get away with putting it in small pot direct. But as a beginner do not try. However, if you get hooked by the hobby, by that time you may be experienced artist already.

Good luck!