It's difficult to say but I think it might be over fed!

No3 has a fair amount of fertiliser in it, and you have added more, so you may have created an imbalance that has stressed the plant.

The weather has been rather overcast of late so minimal photosynthesis will have taken place so the leaves may have held more nutrients than they could cope with and blackened then subsequently fallen off.

The woodwork seems OK so I don't think you have lost it.

I would just accept that it has gone into winter hibernation a bit early, perhaps the winter snow/ rain will cause the surplus fertiliser to leech out and it will kick into growth in the spring.

Are there any new buds forming?

By the way I haven't experienced a situation like this before, meaning, the above details are just a feasible opinion based on the info given!

I keep coming back to this as I find it fascinating its so green yet leafless.

Hmmm thanks, I shall keep googling as well. There are some buds (top right corner of photo) and it has two small flowers on it that are ok, but no leaves!

I will make sure I dont feed it for a while and hope for the best! I have another rose (patio) planted at the same time in the same way, and that one is blooming at the moment.

I still wonder if this was a very bad attack of black spot? As you recently bought it why not take it back?