Could somebody help with care for these bonsia

[Shibui]

do you think it's a good idea to cut back any dead parts or shall I just keep on top of the watering and fertiliser for now.

Dead parts may look unsightly but will not harm the tree. I would normally leave them intact for a while until I am sure those parts are really dead, until (fingers crossed) some of that recovers, until I can easily tell where the live part ends and/or to keep an eye on whether the problem is getting worse.
However, if you can't stand to look at the problem, cutting back will rarely do any more damage.

There is not much difference either way so I leave that decision up to you.

 

[shane0]

To be honest mate after you mentioned the fertiliser I went and got what you recommended and the trees are looking alot more greener which has cheered me up so I might just snip away any dead needles. It's all a learning curve for me but I've got another 60 years to master this art

 

[shane0]

Dead parts may look unsightly but will not harm the tree. I would normally leave them intact for a while until I am sure those parts are really dead, until (fingers crossed) some of that recovers, until I can easily tell where the live part ends and/or to keep an eye on whether the problem is getting worse.
However, if you can't stand to look at the problem, cutting back will rarely do any more damage.

There is not much difference either way so I leave that decision up to you.

I think I've discovered the problem, someone in the area must have bought some cats and I've noticed 2 walking through the garden perimeter and last week I've had soil all over the floor when I've woke up so I think their marking territory ON MY TREES I've bought a couple of cat sensors and placed them around so hopefully they go away. Apparently cat piss can burn the roots?

 

[rockm]

I think I've discovered the problem, someone in the area must have bought some cats and I've noticed 2 walking through the garden perimeter and last week I've had soil all over the floor when I've woke up so I think their marking territory ON MY TREES I've bought a couple of cat sensors and placed them around so hopefully they go away. Apparently cat piss can burn the roots?

yes. That can be a big problem. Cat urine is very acidic and can burn roots. In containers, it's concentrated and hits all or most of the root mass (as opposed to in ground where roots have more space). Flush the soil with A LOT of water. Fishing line criss crossing among the pots and area where they are can keep the cats from bothering your trees.

 

[Shibui]

Apparently cat piss can burn the roots?

It can
But, in 40 years of growing bonsai, I have seen many people blame problems on any external factors rather than accepting their care is possibly responsible.
I would look very carefully at your own care regime before looking at external influences.
Watering , repotting, soil mix, position, sunlight and so many other factors affect pot plant growth it can be hard to decide what is ultimately responsible for success or failure. IMHO the cats are a scapegoat in this.

 

[Betula1]

The shoot die back in your original photos doesnt really resemble the pattern of cat urine spray, and its too far from ground level in these tall pots.
As previously stated it most likely originates from earlier hard clipping/topiary done at the original EU nursery in preparation for Chelsea flower show. Unless the problem is seriously worsening you are best waiting till new growth emerges and if necessary trim dead foliage away carefully. Have the trees produced any new extension growth yet?