I use those plastic trays that you get soft fruit in - they have holes in them already. Or I just use my half-trays from the greenhouse - with seed compost. Don't forget to read the seedpackets, as seeds germinate at different temperatures.

Seed compost with perlite or vermiculite works well for me. I would not bother with dahlias from seed if you want to see quick results they will take a few years to form flowering size tubers.Bellflowers will need some stratification have a look at the link below

http://www.strangewonderfulthings.com/tips104-seeds.htm

Just to conflict above...Dahlias are great from seed and will produce superb, long flowering plants in a very short time. They can either be stored for the winter or thrown away and start again next year. They are also very easy to grow from seed.

Ive also grown several species of bellflower without stratification, Campanula pyramidalis was superb. very easy to grow from seed again.

Good luck

I have had the same experience as Andy J and the dahlias are now 3 yrs old with good tubers now.
I've had excellent success with the cone flowers too. I use a mix of multipurpose and vermiculite to sow seeds. Peat pots were an expensive mistake for some beans I grew a few years back.

I alos put an inch layer of basic compost in the propagator too, for supplying water at a regular rate, rather than trying to water the seedlings.

Spritz is right about the temperature thing too. Make sure you cover the seed to the correct depth too.

My Dahlia's have also done well..it did state they were Annuals,but even after the first year,I was amazed how big,and how many tubers there were,at the end of the season..they just need to dry out before storing for the following year..if you don't want to bother,as it can be hit and miss,and they are prone to going mouldy if not stored correctly in a dry place..and checked on a regular basis,you will still have lots of seeds left to sow again the year after..good luck..

A lot of people grow 'Bishops' Children' Dahlias from seed - very successfully. You can either treat them as annuals, or dig them up for next year, as Bloomer says.

Don't sow too many seeds! The little plastic cups will not take up much space but once the seeds are growing you will need to pot them on into cells or individual pots/cups. You are starting with four pots of seeds, think of where you are going to put the seedlings if you get 20 in each seed pot.