tiling 600x300 porcelains onto plasterboard. just wondering why it has to be single part flexible does it have to be flexible? and cant be cheaper rapid set or whateva is on offer? cheers

You've answered your own question really, plasterboard, having some give will require the addy and grout to flex with it.

really the "cheaper" rapid-set adhesive is only for tiling onto subtrates like concrete that will never really have any movement! flexible adhesive allows for small amounts of movement i.e plasterboard walls that are only held up by stud partition and have no support between the studs

SPF/single part flexible is just terminolgy for a flexible adhesive that just needs water to mix and not a 2 part mix that uses powder and latex additive.

SPF can be standard setting or rapid setting...

But still flexible though, as in 'just' needs water as the admix is already present, and as you so rightly say, can be both rapid and normal setting

yep. so even thought there is noone going to be walking on the walls causing it to deflect etc then it still needs to be flexible addy n flexi grout?. would it be more for vibration and shrinkage than anything else?

You don't have to use flexi but i do as it allows for any thermal changes better.

closing the door is enough to cause deflection