B&Q worktops are rubbish and exceptionally poor value for money compared to Magnet/Howdens etc.
If you're not particularly handy, you are indeed wise to get a man in. If the walls aren't bang on square, it's easy to make a hash of it.
Ring a few guys, some will give you a ballpark figure over the phone or price per joint/cutout. There are also a few outfits that specialise in worktops and cornices etc.
I'd charge for a full day, plus a router bit and colourfil, silicone, contact adhesive etc. You should be looking in the range of £140-200 depending on location