I have fitted elka solid many times before and something that elka states both on the fitting instructions and on there website is that solid hardwood floors should never be fitted floating. I would take up the hardboard and fit ply screwed at 300mm centres, then I would secret nail or glue...
depends on the amount of use it gets also it does not always add value to your house laminate is very hard wearing engineered is not scratches very easily
the easiest rule of thumb is a 10mm gap all the way around even through door ways b&q say them sizes so that if the floor does blow they ask if you went to there instructions if not then they wont sort it as for edging either use the matching scotia or go for white
laminate is alot more hard wearing but i would not get it from screwfix your local shop will sell it alot cheaper and depending on your budget get the best you can afford personally I think that the quick step range is the biggest and best
you will find that the floor is made up from chipboard glued together to form a big sheet underneath would be polystyrene that is why it is called a floating floor
all pretreated decking is pressure treated so that you don't need to paint the ends of the boards as the treatment is forced into the boards at very high pressure so it goes all the way through the boards
the easiest way would be to take the legs off the units as they should be screwed to the wall then screed the whole floor with a flexi screed and level it up that way
what the problem sounds like to me is that under floor is not level it runs of at that point did you check the floor prior to laying the floor or the boards are cupped the problem you get with cheaper end products
I am no proffessional when it comes to varnish but I think the problem is that the floor varnish that you used is a water based product and as soon as the water gets on it it causes a reaction I would personally sand it back down then stain again then apply yacht varnish thats only my opinion
when fitting engineered floors that are glued on the t & g you need to used straps strap them up then using a tapping block and hammer tap the boards together the tension in the straps should help to pull the floor together