So it seems my poly tunnel plans have to be shelved whilst my association reconsiders its 'structure' size limitations - next Spring!
When I took over my plot last autumn I discovered a cold frame the full width of my plot (20ft) and 4 ft wide. Like most allotments this has had a succession of tenants who were not too good at growing and tending plants - or the plot ! The cold frame had obviously had various packets of seed dumped in it in foot long drills with the expected over-population and leggy results. At one end there was an abundance of bag compost obviously added at times - for the seeds presumably. The sides are about 10 inches and mostly made from sleeper dimensioned timber. But there was no transparent cover of any type.
Although this is in a sunny position facing west, I decided that due to its unknown history I levelled out the compost end etc., dug it over and planted a single row of first early spuds to give the soil a break. (Maris Bard, which is a favourite of mine I have often grown when a farm manager).
So when the spuds are harvested I will have a 'clean sheet' - given that haulm and weeds are disposed of. I can raise one long edge to put a tilt on the new transparent roof which I would prefer to make from 10mm twin wall polycarb.
So what would you suggest I should do to make the roof if there are better alternatives (please, not old glass window frames). Should I bottom it out, and lay mulch fabric ready to grow in containers. Or should I use the soil 'asis' given that it has some humus and been given a 'rest' from its haphazard past. If that is the case then I would be happy to at least divide the frame into three zones to allow a rotation of crops.
My intention is to bring-on seedlings for planting out on the main plot, grow salads with extended season, and grow other food plants which might benefit from the conditions under cover. I also use the allotment to raise plants and cut flowers for my home and my wife's courtyard garden. So I can use it for cuttings and 'resting' plants etc.
My sincere thanks to those who can offer advice.