Hmmm, these temperatures are inside the greenhouse, right? I just looked at a weather report for Wales and it looks like ambient temperatures are upper 50's for highs and 40's for lows. That 100F+ I would think was too hot. Did it hit that high very often? Three weeks out for the pepper seeds seems to be stretching it a bit, give'em another week if nothing happens I'd re-seed.
In the trays where the tomatoes have sprouted, are the empty cells where tomatoes did not germinate? Tomatoes germinate faster/easier than pepper seed. If you had that entire tray seeded with tomato seeds then something about the germination environment may have been bad.
40F is not too terribly low of a temperature and with the mass of soil in there some heat from the day should be retained. Unless I was absolutely sure I could get to the hot house and open the vents on it I think that I would feel better leaving the vent open a small amount all the time...and open it wide during the day.
The way that I germinate seeds is to have my soil container covered in plastic. I actually use a plastic pan with styrofoam cups in it to start my seeds in. I cover this with a loose sheet of thin plastic...enough to hold most of the moisture in but still allow a small amount of air to enter (very small amount of air). I add some water maybe every two or three days. The trays basically stay at room temperature.
