without knowing what your hatch is like as you dont have pictures or a description (with ladder?, -on hinge?, just a board? ) i would say either use traditional insulation in a bin bag over the hatch, or use something like THIS .
also use some draught excluder stuff around the hatch.
Be careful with this DIY plastic bag around insulation material and fixing it to the loft hatch.
Plastic is flammable and can both start and spread a fire. Insurance companies have all sorts of reasons not to pay claims including valid ones-e.g. defective materials that don't comply with any BS standards or building regs.
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