For outside use I prefer to use shielded cabling. Depending on the situation sometimes even in a pipe. Normal cabling is just too easily interfered with (both physically and electronically). I've seen rats and squirrels eat the cladding of cables. Stone/house martens are well known to do this as well (I am told they really like the oils in the plastics). So, it doesn't have to be human error/carelessness either.

Before you start doing things that take a lot of time/effort/money I'd recommend you buy a cheap UTP signal tester first. This will help eliminate or identify the cable or the RJ-45 connectors as the cause of your problem. You can get them for about $10 on Amazon or other such sites. They won't be Fluke quality, but more than enough to do simple testing.

Only if testing shows problems with the signal I would replace the cable. Chances are it is not the cable but something else if you already did a visual inspection of the cable. Mind you though, sometimes you won't see a small breach that would be enough for moisture to get in.

[Edit] I had to do some searching on Amazon for you, but here's a link. I have had this one in my field bag for about 10 years now:

https://www.amazon.co.uk/Network-Cable-Tester-RJ45-RJ11/dp/B08JQ6J6XZ/ref=sr_1_32?keywords=utp+cable+tester&sr=8-32

Like I wrote, definitely no Fluke. But good enough for quick and dirty checks before going into full diagnosis mode.

Good luck in tracking down the causes of your problem.

Thanks. The cable I got was expensive because it has a gel inside that rodents don't like because it tastes bitter.

I just bought a tester.

And I also saw that another outdoor cable is just £30 on amazon so that's another option if it is faulty.

A couple of months ago, I put the wifi access point in the house and it worked fine. And then relocating in the shed I couldn't reproduce the fault. The long ethernet run used to always work with the laptop plugged into it, but yesterday it didn't for the first time. It's been ongoing for quite sometime so hoping to fix this soon

For outside use I prefer to use shielded cabling. Depending on the situation sometimes even in a pipe. Normal cabling is just too easily interfered with (both physically and electronically). I've seen rats and squirrels eat the cladding of cables. Stone/house martens are well known to do this as well (I am told they really like the oils in the plastics). So, it doesn't have to be human error/carelessness either.

Before you start doing things that take a lot of time/effort/money I'd recommend you buy a cheap UTP signal tester first. This will help eliminate or identify the cable or the RJ-45 connectors as the cause of your problem. You can get them for about $10 on Amazon or other such sites. They won't be Fluke quality, but more than enough to do simple testing.

Only if testing shows problems with the signal I would replace the cable. Chances are it is not the cable but something else if you already did a visual inspection of the cable. Mind you though, sometimes you won't see a small breach that would be enough for moisture to get in.

[Edit] I had to do some searching on Amazon for you, but here's a link. I have had this one in my field bag for about 10 years now:

https://www.amazon.co.uk/Network-Cable-Tester-RJ45-RJ11/dp/B08JQ6J6XZ/ref=sr_1_32?keywords=utp+cable+tester&sr=8-32

Like I wrote, definitely no Fluke. But good enough for quick and dirty checks before going into full diagnosis mode.

Good luck in tracking down the causes of your problem.

I use the Klein equivalent every time I reterminate an RJ-45 cable.  It's saved me from making bad cables numerous times.  And for a quick diagnostic, throw out a known-good Ethernet cable across the lawn and connect the router to the access point using it.  If the access point comes up on the substitute cable, the buried cable is at fault, and you can replace it. 

I use the

Klein Tools VDV526-200 Cable Tester, LAN Scout Jr. 2 Ethernet Cable Tester

for testing Ethernet cables.   It includes an adapter to put at the end of the cable (loopback I think) so you don't need access to both ends at the same place.

Bob

Thanks for all the replies.

I think I found the issue just now. I happened to notice that there was play in the ethernet jack in this network socket.
https://www.amazon.co.uk/gp/product/B07G2CKPT4/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&p&sc=1

It's not a great design as any cable movement can put stress directly on to the soldered terminals.

I can replicate the issue by flexing the jack so I think I will replace with something a bit stronger - anyone have any recommendations for a reasonably priced surface mount network socket?

You might want to check out this product.  It will work with CAT6.