Thanks for the replies. It is 30mm thick granite, I think the picture distorts the depth. I have spoken to the fitter this morning and he's agreed to fix it - narrowing the gap and darkening the filler. The slabs were from the same batch but there is a difference in the texture from one side to the other of each slab (we picked them in the warehouse). He should have laid the right hand one around the other way, but the grain would still have been at right angles across the seam. In the end that is in a less used corner of the kitchen and we are not so concerned about the lack of matching of the granite - mainly because I don't think there is a good solution.

No it shouldn't be able to move around, the fitter/electrician who installed it didn't bother to use the fixing clips supplied, probably because the worktop is 30mm thick and the clips are designed for 40mm tops. Did they also fit the sealing gasket to the hob? Is this why the hob sits a few mm above the worktop. You need it fitted tight to the worktop so any spillages don't leak into the cupboard below and a having water damaged cabinet, if you have a built under oven below then you definitely don't want any spillages dripping on that!

Quick and easy way to get a comparison quote, look up a quote on DIY kitchens... you put in the size choose the worktop, cut, cut outs etc and then it gives you a price there and then. That sounds reasonable to me though. We’re going with a silestone quartz. Has a very subtle marble. You can also order samples from DIY to look at them or do tests re staining etc. There’s also a Houzz story somewhere about different kitchen worktops! :-)

I know granite is porous, so I wonder if a substance like vinegar or lemon juice was accidentally left on there for long enough to cause damage? Was the granite sealed after installation? I wonder if it could be sanded down then re sealed.