Guitar now plays perfect.  Shame I don't.

Do you know the radius of the fingerboard?
Lots of vintage guitars, with a small radius (7.5 - 9.5") will "fret out" on bent notes, esp. above the 12th fret. That is whay newer guitars have a larger radius (12") or a compound radius (12-16"). That gives you a flatter fretboard overall and with a compouond radius, it gets even flatter as you move up the neck. Though, first position chords are more comfortable on a smaller radius neck.

If I had to bet, I would guess that one of the higher frets was not properly dressed at the factory, or was not fully seated. If you see a fret (above the 18th) with a tiny visible gap between it and the board, you could try tapping it gently with a plastic headed hammer to seat it. If you sight down the neck, from the bridge end, and see a fret that is too high (proud), you can block sand it with 320, 400 & 600. That will make the top flat, so you'll need to re-round it with fine sand paper or synthetic steel wool without out a block.

Wibble mentioned that it is a Epi LP studio model which means it has a t-o-m bridge which is a 12 deg radius. The only way to change the radius on a t-o-m is to buy blank saddles and recut to the given radius you desire. The frets may be high and or also the neck may need to be adjusted. The problem with sanding the fret is that if you don't use a radius block the fret will not have the same radius as the rest of the neck.

Bootlegger.

If I had to bet, I would guess that one of the higher frets was not properly dressed at the factory, or was not fully seated. If you see a fret (above the 18th) with a tiny visible gap between it and the board, you could try tapping it gently with a plastic headed hammer to seat it. If you sight down the neck, from the bridge end, and see a fret that is too high (proud), you can block sand it with 320, 400 & 600. That will make the top flat, so you'll need to re-round it with fine sand paper or synthetic steel wool without out a block.

Tried it in all 3 selector positions and still got the same thing. I assumed it was possibly the positioning of my pickups themself (maybe need raising closer or farther away from the strings) or possibly the general setup as its been sat in a gigbag for a couple months - Reckon it'd be an idea to take it to a guitar tech and get them to do the biz for me?

Which pickup is active when this happens?  It might be that you are bending the string beyond the field of the pickup IF it is the neck.
Another thing is that after a couple years of "storage" my Epi ST275 selector switch was giving me intermittant contact in some positions that would go in and out with just a slap to the body....  cleaned the switch out with a shot of good old WD-40 and a shot of air from a can of PC duster, and it's been fine ever since.

Take Care;  Doug

Bit of a tech question i'm hoping someone can help with.

I got an Epiphone Les Paul Studio using Rotosound Purple strings and I've just noticed if a do a bend on my high E string (For example - Bending the A at the 18th fret) then the sound cuts out after about a quarter bend. Not used it in a while so wondering if anyone has any idea's.