I'm brand new to home brewing and have bought a starter kit and a box of Norfolk Wherry. We are 1 day into our 6 day first ferment, and this morning my fermenting tub was whistling because there was so much gas built up under the lid. I have let it out, for fear of the lid shooting off and exploding everywhere.... Am now concerned that I may have ruined the beer?? Any advice would be greatly received
You have not ruined the beer. If anything early in the fermentation, day 1-2 letting in a little oxygen is good for the yeast as it allows them to form more flexible cell walls. As Yeast require oxygen to create sterols which maintain cell wall flexibility.
Usually you want to leave the lid covering but not locked down for day 1-2, to stop CO2 pressure build up. Then end of day 2 clip it down and leave it to finish.
Also, it says 6 days for fermentation, I would usually leave it for 10 to ensure it is fully fermented out. To do this properly you should take a gravity reading, but if you are too busy or unsure then a few extra days in the fermenter won't hurt and will ensure it is fermented out.
Beer can be fermented in an open barrel. I don't totally recommend it for the faint hearted but the Belgian Lambic brewers do this. A vent should be left in a fermentation vessel, eg a bubble trap or just leave the lid unclipped SLIGHTLY - you don't want flies in there! I would not recommend clipping down the lid before a week. and then check on the pressure daily. Allowing air to come into contact with beer is not such a terrible thing. To aid clearing I pour my beer from a 25 litre bucket into another in air. Lots of foaming and swoshing. Then I put the lid on and leave it for some more days until it has dropped bright. Then I pour it back onto a solution of the priming sugar (with foaming and swoshing) before bottling or kegging. So if I can do that (and I do it regularly) then letting the gas out is probably of no consequence at all. I wouldn't worry.
It is best if beer is left for a longer rather than a shorter time to condition. IMHO Beer should be about 14 days (and longer like lager) in various primary fermentations and then at least 14 days in secondary (carbonation) fermentation/conditioning. Leaving it for a month to condition would not be wrong. One can certainly drink young/mild beer after some days but it improves greatly with some maturing over at least a fortnight.