Having insulated the attic at 1st floor ceiling level and then leaving the attic open to stop your services freezing defeats the purpose of the insulation.
This is why I advise people to consider insulating by following the line of the roof with the insulation, ventilating in accordance with http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,1647,en.pdf (Diagram 11, P. 28 TGD F 2009, Ventilation).
The services should therefore lie within the insulated volume and be protected from the worst ravages of the winter freeze.
However in the case of a house of this vintage, the kind and positioning of the insulation should be carefully considered - and its not something I would advise on off the cuff through AAM.
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Working within your current set up, the simplest suggestion on AAM last year was to install a heat source in the attic for the coldest days - this is the light bulb solution you said you didn't want to install.
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Leaving your attic trapdoor open is a good way of increasing your heating bills and causing draughts all around the house, but it should help prevent pipes freezing.
However even this is not a foolproof method since heat rises and there is no guarantee the air will circulate to all part of the attic pipes in which pipes may be laid.
One thing you must be sure of is making sure there is no insulation under your water tank(s) and making sure the insulation at ceiling level joins with the insulation on the side of the tank. Your tank should have an insulated cover and all pipes to and from it should be lagged for their full length.
Yet in your attic your roof has also lost all its windproofing - the snots of mortar filling the gaps between the slates - without these your roof is basically an "open" structure through which air can move at all levels and which will freeze more readily in cold and windy weather.
My best advice is to have your house re-roofed with a well-considered weathering and insulation solution.
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Heating and attic is a simple thing, but tracing the routes of pipes and ensuring they are proofed against harm from cold ingress is less simple.
"As far as I cam see..." suggests that you are not fully aware of all the service routes in your house.
There may be risers and drops in the house in inaccessible areas and in corners which are currently cold bridges - if unlagged, these pipes can also freeze.
ONQ.
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All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matter at hand.