Thought I would provide an update if anyone was interested, the plasterer has refunded me £90, out of a good will gesture, as he put it! A fraction of what this will cost me to put right including my time.
He said it was a joke for me to blame him! His cause to the problem was trying to blame the brand new, securely attached plasterboard, saying it was pulling away and that it also could be to do with the weight of the thistle on the plasterboard pulling away... which he applied, but was denying that was his fault... which makes no sense at all.
I have spent hours messaging him and sending him pictures of his work and his responses are unlogical and not worth anymore of my time.
I since have ripped out the old plasterboard and am going to reattach new plasterboard, tape and join myself and then paint, as the thought of hiring someone again to plaster seems like a gamble I dont want to take. I sent him pictures to show everything was bonded correctly, which throws his excuses out the window, so he most likely won't reply. I will attach some more pictures of this.
Thanks everyone for your feedback and taking time to reply, it is much appreciated and really helped, its nice to be able to get second opinions like this.