So, this is going to sound obvious but ...

My pattern now says (and I think it's ambiguous).

Front
Work as given for Back to **. (I've just finished that: 48 cms of stocking stitch).

Shape Neck -
Next row - K57, turn. (yep, done this far)
Cont on these 57 sts for left side of neck.
Dec one st at neck edge in every foll alt row until 48 sts rem then in every full 4th row until 40 sts. rem.

What the feathers??

So I knitted 57 stitches and I 'turned' ... does this mean my next row is purl heading back in the direction I'd started with the knit row (but half way along my cast on 138 sts?) I'm confused!!

Yes, you've got it. Turn after k57 just as you would if you were at the end of the row and purl back on the same 57sts. You're going to finish the left side of the neck and left shoulder, then go back to the remaining sts which are hanging out on the needle (or on a stitch holder if you prefer).

dear wonderful salmonmac

Yes - I did put the other stitches onto a stitch holder as I had one and wanted to use it.

So I'm reading the pattern that the decreases and done every PURL row - is that right; so I purl the first two stitches and slip the first one off to perform the dec

Sorry - slightly confusing.

Here's the pattern exactly as written (with none of my italic comments):

FRONT
Work as given for Back to **.
Work a fruther 18 rows stocking st.
Shape Neck
Next row K57, turn.
Cont on these 57 sts for left
side of neck.
Dec one st at neck edge in every foll alt row until
48 sts rem, then in every foll 4th
row until 40 sts rem.
Work 3 rows.

I would purl back on the 57sts, turn, knit to 3sts from the end (the neck edge) and then k2tog, k1. It'll give you a nice line of decreases up the neck edge. When you get to the other side of the neck and shoulder, the sts that are hanging out on your st holder, you can mirror this decrease with a k1, ssk so that the decreases line up along the neck edge and slant nicely up to the left.
If you want to do a bind off at the edge as you're suggesting, yes, you would do it at the beginning of the purl row. It'll probably be covered by the collar or neckband anyway.