Update: I'm in the middle of doing stuff and am not getting the time I need to finish the job, but here's a run down so far. Maybe someone can advise if they have any solutions later on.
First up, I installed a fresh copy of High Sierra onto the SSD and switched the boot order so the thing is a whole lot faster now and running smooth. Very simple process and pretty painless. I would recommend this to anyone with these slow HDDs, it really is a super smooth process and the results are fantastic.
That's for you Titi, easy peasy.
BUT, where I sit now and what I have to do is a bit more complicated. It would be easy to format the HDD and use it as storage, but this machine is first and foremost used for photography and it has an existing photo library I need to keep and use, so I need to have Lightroom and Photoshop moved onto the SDD before doing this.
So before touching or changing anything on the HDD I downloaded and installed the Adobe Application Manager and then did a fresh install of Lightroom and Photoshop to the SSD. So far so good. I pointed new Lightroom towards the existing library on the HDD and it all worked fine. First problem popped into my head, if I want to format HDD, then I have to rethink this and move the library temporarily and back again after format.
Next problem is that it's now not possible to open Photoshop directly from Lightroom for editing. I've had quick google search and there are several reasons why. No doubt it's to do with Library being on a different drive from the apps. I'll need to look at that in the next day or two but there are two problems that I need to fix if I am to run the apps on one drive, and the library on another.
So that's where I sit now. I had thought just to stop what I'm doing, format the SSD, clone the HDD onto it and then find a way to move the library to the HDD again. Does anyone know if this will work and will I just have the same problems again? Library is about 300gb.