Being the ingenious ex roadie that I am (in my opinion, you can fix just about anything with a ratchet strap and gaffer tape), I thought about how an optocoupler works. Basically it is an infrared led that shines onto an infrared sensitive transistor.
Hmmm I think. How about a light dependent resistor with an led? Sounds like a plan!!!
(Technically, this isn't an optocoupler, but it 'optically separates one circuit from the other', so it's good enough for me... at least 'till the shop opens.)
Step 1: Parts you'll need
I used an ultrabright white led that i had bought as a job lot for another project that didn't get off the ground yet (30 quid for 1000. Bargain!).
LIGHT DEPENDENT RESISTOR
I used one that i had in the parts bin (less than a quid)
HEAT SHRINK SLEEVE
I've got a box of the stuff that i picked up from the army surplus store.