Thank Scouse.
I have just pulled out my flasher unit and it is a YWT10003. As you say it is a 3 pin.
So all this time I was thinking that the previous owner put it in as a way to stop the bullbar indicators from flashing the trailer dashboard light and it is probably standard.
My HR towbar is definately aftermarket as the trailer harness was wired in with scotchlocks into the LHS taillight with the RHS indicator being wired into the RHS bumper light. A bit of extra work since they could have just used the factory trailer loom (which I've since rewired it to).
An interesting note is that it has this written on it:
21wx2 + 5w + 1.2w
MAX 21wx4 + 5wx2 + 1.2wx2
So does this mean the side lights are only 5w? As it would overload the unit if all six indicators were 21w (when using the hazards of course as indicating only uses 3 lights at once).

