DIY casror adjustment

Anyone any ideas for checking and adjusting the front castor on my Spider. It is probably miles out since taking the suspension to bits, forgot to measure the arm length before messing about with it. None of the garages around here seem able to, or don't want too.

Odd that a garage wouldn't want to do an alignment. No matter. First, a few things about caster; it is for directional stability, aids in returning the steering wheel to center after rounding a corner, affects the effort required to turn the steering wheel (more positive caster makes for harder steering), a vehicle will pull to the side that has the more negative caster and caster is not a tire wearing angle.
You could start by adjusting the lengths of the caster strut rods (the rod from the upper wishbone going forward) so the the upper wishbone is approximately centered in the slot in the inner wing. Air up the tyres and drive the car on a straight and level road. Loosen your grip on the wheel and see if the car pulls to one side or the other. If, for example, the car pulls to the left, the left side has more negative caster than the right side. Now a choice has to be made. If the steering effort is high, shorten the caster strut rod on the right side. If the effort is low, or if the car wanders, lenghten the rod on the left. Repeat until the car tracks straight.
Changing the caster will also change the toe so this will need to be adjusted as well. I'll post a link on the toe adjustment, if I can find it.

Here's one link...

...and another.

Keep in mind that the procedure in the first link is for a LHD car. For a RHD, start with the right side and ADD 5mm to the length of the left tierod.

Thanks for that Papajam.