Further searching and my control unit looks to be one of the Commander 3000 Classics from this listhttp://www.hbmmercruiserparts.co.uk/pro ... tNo=889892
This leaves me with 2 possible switches -
3 87 18286A32 SWITCH ASSEMBLY, Trim/Trailer 1 superseded by 87-18286T32
superseding part 87-18286T32 SWITCH ASSY £45.10 £54.12
3 87 18286A31 SWITCH ASSEMBLY, Trim/Trailer 1 superseded by 87-18286T31
superseding part 87-18286T31 DESC SWITCH £45.10 £54.12
I'm really quite shocked at the price of this little switch, especially when they sell them in the US for $27 - http://www.boatstore.com/mercury-quicks ... 86a32.html
Still searching for a better price...