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replace the door seal which seems to have been crushed by the latch, though not sure that won't just happen again.
It looks like the squashed/cut away part is where a separate deadbolt keep may be on the frame. It looks similar to an old GU locking mechanism and two part centre keeps are common with them.
If the keeps on the door frame are two separate pieces for the (upper)latch and (lower) deadbolt , the seal may be compressed there, if the deadbolt keep is proud of the frame. In which case it will happen again in the future, meanwhile the extra compression will put more pressure on the locking mechanism.
Trap a thin sheet of paper or plastic in the door when you close and lock it. If the paper needs a slight tug to remove it, I would be inclined to leave it. If the paper slides ip and down easily and offers no resistance to being pulled out, definitely look at adjustment of the door and mechanism, then maybe replace the seal.
user69121-
Thanks, they're one piece, definitely slightly proud of the frame. I feel like it may be missing a part as the latch actually does nothing and I have to crack the handle up (which presumably extends the deadbolt?) To get it to stay closed). Might be another case GSI (get someone in) as I think the door has sagged slightly and my back door needs some attention too... ahem. At current DIY rates I'll not get to it until spring and then likely forget about it again.
Polygon
Found similar in mine the other day leaving it there as most of it is boarded out now and fuck moving insulation around, horrible job.
Small win, finally got around to foaming the big gap around our door as it became very obvious that cold air was pouring in. Just have to sort out the utility meter cavity now which appears to have a whole brick missing where it joins the door and replace the door seal which seems to have been crushed by the latch, though not sure that won't just happen again.
Getting a bloke in to fit vented caps to the disused chimneys too, mostly to prevent water ingress though I'm hoping it's going to reduce draughtyness further.