Redbeard

All my thoughts as I read the posts so far pre-supposed that you would be dropping the plasterboard and working from below. On the basis that you can't see what you don't see I would not like to guarantee how well you can seal the joints. Variables include rafter pairs which are not a completely consistent distance apart, rafters 'on the twist' and so on. I have never seen a sloping soffit insulation job done 'from the apex void' where it has been completely tight. Even when access is available it's not necessarily wonderful. Many rooms-in-the-roof insulated from above when re-roofing are testament to this. For an excellent job I would kit up with PIR,(if that is your material of choice - others are available - air-tightness tape and a/t primer, bite the bullet, drop the ceilings (sorry!), clear up the mess and insulate the slope leaving a minimum 50mm ventilation gap below the felt. 0versail the sloping insulation past the flat 'loft floor' and carefully detail the junction between PIR and 'fluff'. Are those 150 rafters?

To comply with current regs it should be at least 27cm deep. 40 or even 50cm is not too much for me, as long as you can get access when necessary.

Yes, possible, but as noted by the OP there seems to be air movement in there, so any solution would require designing out unintentional ventilation and designing in the intentional. I suspect the OP was hoping to avoid dropping the ceiling. I'd go the whole hog, but with new 'proper' ventilation designed in.

We bought ours online. My notes tell me it was Floormart.co.uk in Leicester. It was 2 or 3 years ago, but their website says they still do Marmoleum. I thought at first they had delivered the wrong colour, but found on querying it that they had wrapped my roll in a Marmoleum offcut to protect it from damage during delivery. They were v. helpful. I don't know if they have a showroom, but they are not near you anyway. Possibly worth giving them a call.

Is the wiring in contact with that EPS insulation when it 'lies down'? If so you want to sort that. EPS leeches the plasticisers out of PVC cable sheathing. How well-built is that structure? Do you think there is any risk that external air will circulate between the EPS and the OSB at the top of the joists? If so, even if it is a Warm roof (with, say, a minimum of 150 PIR above) the 'Warm' roof element may be doing very little. Edit: Sorry, I had not read @Mr Punter's post. I'd still suggest satisfying yourself re (lack of) air-leakage, though, or the 'by-pass' possibility still applies.

Thanks @sgt_woulds. I was assuming VCL on sub-base and PIR or similar non-breathable ins'n. Good clarification though re WF.

No idea how I came to answer a 2-year-old post! Nevertheless if anyone is interested in the life of (some) sleepers note that the first '10' above was a misprint. It should read 5 (max) years left.

My first thought was that in a Warm Roof (note the initial capitals) you don't need vents - indeed there isn't anywhere to vent unless you want complete thermal by-pass. Then I wondered if by 'warm roof' you mean insulated between and perhaps also under the rafters, giving you a roof void which is warm (and a heated volume which is greater). A true Warm Roof has all the insulation above the rafters. BUT if you are simply talking about the oversail and the rafter ends then maybe there is some argument for ventilation there. However I have doubts as to whether that air will flow enough to purge the area of vapour. Ventilation generally needs 'ins' AND 'outs'. Diagonally-opposite is always good. If I have completely misunderstood please ignore all of this!

How long are you planning to live there? I had treated softwood sleepers laid to retain approx 700mm soil around 10 years ago. They are noticeably rotting and maybe have 10 years left. I am now 10 years older and wish I had used masonry.

Welcome. I'd suggest dense, breathable insulants such as rigid wood-fibre or cork. You/we will need to try to build up a 'picture' of the 'slightly moist' nature of the marl. Breathable insulants can be a help to breathable walls. They are not an instant solution to wet walls. If this is a conversion, how are the internals finished? Plaster? If so, gypsum or lime? Plasterboard? Is it insulated or not?

Tiles?

How deep do you have to dig? Assuming you maybe need to excavate the connection, take out a 'T' joint and replace a straight section the digging is probably the worst bit. Assuming the drain is circa 100mm you can get sleeves with jubilee clips to span the gap (or 2 smaller sleeves and an infill bit of pipe if it's a wider gap). Cross-posted with @ProDave.

No. I have had a couple of situations where (I cannot remember quite why) I was a little suspicious, and had a sample tested to be on the safe side, but NADIS (no asbestos detected in sample) was the answer.

Do you mean asbestos fibres as 'long hairs'? If so, no. I am not, definitely not, an asbestos expert; just someone who is unlucky enough to have encountered it quite often in the course of my work. Don't forget the PPE when you take your sample!

Check the HSE guidance before you take the sample.