It's usually a good presumption that you get what you pay for. Silestone and Caeserstone have a long history and great reputation. There are some newer options but not tried and tested yet. I think it depends on your budget. Trends have moved away from dark worktops I feel having worked in kitchen design. We certainly did not get a lot of requests for it. See if you can reuse the granite in a bathroom maybe or a utility counter if you decide to replace it. Cararra style quartz is beautiful. Ask yourself these questions first. Is this your forever home? What else could you use the worktop budget for? Does this only affect you in winter or darker weather? Could you lighten the kitchen some other way? Bigger windows, lighter units, better lighting etc. Your budget, your choice. I think replacing should be quite simple as long as the thickness is the same. If you have upstands expect to do some making good.

No, not all quartzes are made equal. There can be a big difference in the overall quality, the resins used and the amount of "stone" content. When it comes to Carrara styles, thee "realism" of Silestone (our "go to") & Caesarstone is generally much better and the whites whiter. Backup support is also much better.

As to the recommendation of any fabricator - at the end of the day, their margin is in the labour & machining costs and they will protect that and view the "slab" cost as just that - a cost. They will protect their margin by "down-selling" you to a lower grade product to keep the overall sale.

At the end of the day it is, of course, your choice - do you save £1000 today and opt for the cheaper, lower quality product or view it as a 20 year investment of just £50 a year.

If it is your "forever" home, go quality. If you only intend to be there a year or two and just want the look, take the saving.

As for replacing the granite - usually straightforward, only slight risk if there are tiles down to the granite and awkward "pinch" points but I've yet to see any major issues arising.

No unfortunately not, there seem to be a lot of far east imports on offer and these may not be suitable for use as a worktop. if you think that you use your worktop for food preperation etc I'd be looking to see if its safe, a quartz that may be used as a floor and wall cladding may not have the same properties needed to be food safe, where as a reputable brand with a website, certificates and a reputation to keep is a safer option. I couldn't find a Cosy Stone website.

Go and get samples and then put red wine on them or anything you use frequently that stains and leave them on the sample’s overnight. See which comes off cleanly and that might help with your choices.