I've just gone through the practise of re-stringing and re-tuning the strat this afternoon with some shiney new Ernie strings. The main problem I have is retuning the lower E as thus far it struggles to get beyond above a B, in fairness i've read a bit on the net already about retuning to a drop-D to get more of an authentic 'rock' sound, so in principle the converted string to B should be sufficient enough to allow some leeway whilst practising?

I'm not sure I understand what you're asking...are you trying to play in drop-D but can't get your string above a B? Sounds like your string is waaaaaay too heavy for what you're trying to do.

I'm not too sure either, it could be just familiarisation with the Ernie-brand strings as before i'd just bought some random music-store strings for a 'temporary' quick fix. Playing some Rae Jepsen currently on the usual G Em C and D chords and the guitar sounds good, so it could just be me being anal about the dropdown from the bottom E to B.

E and B is a huge difference. Keep it that way and you're going to wreck your intonation.

What gauge strings are you using?

You can't get the low-e string higher than a B? Are you sure you put it on correctly? That really really shouldn't be an issue. If you can post a picture of the guitar (headstock and bridge especially) that might be helpful.

Yeah, OP, it's confusing. On the one hand it sounds like you're trying to tune to standard and can't get the string to wind past B, which means to me the string is put on too tight and needs to be loosened around the peg. And on the other hand it's just .. stultifying because I can't imagine what you mean (or why you'd want your E string at B to begin with. Just sounds like a flat tire to me) ..