UKH

What is the best stuff to fill/seal a long vertical gap that has opened up between two external walls? We have a small extension that has subsided a bit opening up a gap Max 1cm.

English dead?

Mastic, usually in cartridges, easy to apply with a gun. 
Usually extensions have deeper foundations than original buildings so could be the house that has subsided. You might have bigger problems if it continues! 

Filler might do it but as d50.... has said it might be something more serious. I'd watch it for a few weeks. Perhaps put something across the gap so you can use to see if it has widened.

If it get bigger then, sorry to say, it could be something filler can't solve.

Dave

Was it not tied into the existing structure? Isn't that a thing?

Think given the location it’s mining subsidence 

Could try.  

ADSHEAD RATCLIFFE ARBOMERIC MP10 POLYMER SEALANT

or Geocel 201 Polymer Sealant is a one part, ready to use, flexible polymer sealant for expansion joints in building construction. It has excellent adhesive and weathering properties and cures to a tough, flexible sealant. 

No it was Jerry built I think, inadequate foundations. A small lean to structure. The gap has widened 3mm in ten years so it's not going anywhere fast. If I had the energy or money I'd knock it down and rebuild but for now I just want it weatherproof.

Stabilise it with Helibar before filling the gap otherwise it’s probably just going to keep opening. 

Can you say that and be PC? 

From the freedictionary.com;

'Although the phrase is widely thought to have come from the British World War I slang term for “German,” it well antedates the 20th century. Some possibilities are an old English word for tumble, Jericho (as in Joshua causing the walls to come tumbling down)'

Not guilty your honour.

That looks just the ticket - thanks.

In terms of filler/sealant - mastic which only cures on the surface is the way to go, silicone will come away from the sides.

10mm is too wide for most sealant, even outdoor masonry stuff if the crack has depth (like a gap between 2 structures) and will slump down. What you could do is get some sort of packing, like a bit of foam pipe insulation (just something) and cut with a very very sharp knife into strips that are a bit wider than the gap. Shove them with force into the gap leaving the surface 1/2cm behind where you want the mastic to go. That should give something for the back of the mastic to stick too and not slump before it has cured. Now gun the mastic into the gap. You could try similar with expanding foam and trim any excess foam back below the surface but that's a right faff.  "Enjoy"

It's called PU backer rod, a cheapo alternative for bike tubeless foamy inserts.

Sorry Tom but what you’re saying is rubbish. 

Let’s note tho op has said 10 mm max gap    I posted that 201 was the type of product that  could be considered  this is from the dater sheet  

@JOINT DESIGN
When designing joints using Geocel® 201, the minimum joint width should be 6mm. For joints between 6mm-12mm wide,
a seal depth of 6mm is required. For joints above 12mm wide,
a width to depth ratio of 2:1 should be used. In situations where fillet joints are needed, a minimum of 6mm sealant bite to each substrate is recommended.

So it says for an approx 10mm wide crack (in range between  6 and 12mm wide) crack a depth of 6mm is required.

I'd recommend 1/2cm in my post and not 6mm. Apologies, I'll consult the datasheet next time as I was 1mm off

That'll be a minimum crack depth, not maximum.