Tracing water ingress.

[Jacob]

Looks like inadequate weather bar altogether. Even if perfect when new it looks doomed to movement and failure.
Not clear from photo but presumably the grey area to the left is outside, and level with wooden threshold?
Given heavy rain and wind, water could pool here and blow up and over the seal. Outside perhaps needs to be more self draining.
Basically a design problem - the detail design is more important than the seal itself.
Is that a weather bar to the right? It should have a drip channel under so that water draining off the front can't blow back across the underside.
Bigger photo could help.

 

[John Brown]

It's worked ok for years, the water ingress is recent, and the seal I'm asking about has split in places.

 

[Jacob]

It's worked ok for years, the water ingress is recent, and the seal I'm asking about has split in places.

Right! That looks OK. I guess it must be the splits in the seal! It's quite possible for water to get blown up against it.
Where does the water show itself on the inside?

 

[Doug71]

They are often all plastic like this.

https://www.sdhardware.co.uk/?i=204...ztW3uAysRymEuFldIe-tK5PVbVHS7f4BoCu7wQAvD_BwE
It's kind of the old fashioned way of doing things and you can get water in if it's the wrong kind of rain in the wrong direction if you know what I mean. Are there any grooves or draught strips in the frame/jambs?

This is what people often use these days, they do give a much better seal.

https://www.marchesironmongery.co.uk/exitex-macclex-15-2-threshold-various-finishes.html

 

[John Brown]

My whole house is old fashioned, being a converted 400 year old barn. I really just want to replace the seal, but don't know what search terms to use. If I need to do major surgery, I'd rather leave it until summer.

 

[Jacob]

My whole house is old fashioned, being a converted 400 year old barn. I really just want to replace the seal, but don't know what search terms to use. If I need to do major surgery, I'd rather leave it until summer.

Probably better to leave the seal until summer. If wet it'll be tight in the slot and the new one won't be so easy to stick/seal etc

 

[John Brown]

Probably better to leave the seal until summer. If wet it'll be tight in the slot and the new one won't be so easy to stick/seal etc

I'd still be interested to find out what it's called, and where to get it!

 

[Jacob]

I'd still be interested to find out what it's called, and where to get it!

It's a "water bar".