I wondered was when I should take the plastic lid off and how?
A simple enough question but not so easy to answer simply because there could many influencing factors as to when. e.g. the light and temperature range you have when you do remove it.
I guess you know that you now want to harden off the seedlings (aclimatise)
To do this you have got to
gradually reduce the temperature to that that your seedlings will ultimately require and what with the vagaries of the UK weather and not knowing the facilities you have to do this, makes it difficult for me to be more precise!
In your case it has been made reasonably easily because of the plants involved i.e. kohl rabi, cauliflower, onion, Lamb's lettuce and sweet peas which are all relatively hardy!
Having said that; I personally think you have been a mite too early apart from the sweet peas and onions, in sowing them, particularly in your neck of the woods.
I say this thinking that you not be planting them out for a few months yet meaning you have to keep them alive for quite some time yet.
Regarding damping off which I agree could be a factor, I don’t think this should be your main concern!
Low temperatures are more likely to create a greater risk.
I try to maintain a minimum temperature of around 40°F 4°C, and in the case of onions this would be around 50°F (10°C)
In the following pictures you will see how I step down my heat levels e.g.
1) Closed propagator
2) Once germinated open propagator vents for a couple of days
3) Place on hot bed and cover with transparent tray cover for a few days
4) Leave on hot bed with covers removed again for a few days.
5) Put on high shelf near light in a frost free location
6) Put into cold frame a few weeks before planting out time.
7) Plant out when all fear of frost has passed.
The time scale for the above operations apart from onions & sweet peas in my case, is about 4-6 weeks from sowing to planting out e.g. I sow my cauliflower week 13 and plant out around week 18/19 depending upon the weather.
So you see if your planting out time is similar to mine you will have to look after them for at least another 14-15 weeks.
I wish you the best of luck but I would suggest you prepare yourself to sow more Lambs lettuce, kohl rabi & cauli in about 10 weeks time.
Let me point you and anyone else for that matter to my set up and methods of sowing in the hope that it will increase your success rates.
I know most people don't have the set up I have but with a bit of imagination you can achieve similar results.
My set up;http://www.thegardenersalmanac.co.uk/Data/My%20set%20up/My%20Set%20up.htm
How I sow my seeds;http://www.thegardenersalmanac.co.uk/Data/Seed%20sowing/Seed%20Sowing.htm
