hot dip galvanising of chassis

Blast and paint is normal.  But depending on condition you may want to media blast, not sand.   It depends on the condition and how much rust etc needs to be removed.   If you dip it the chances are it will last longer.  I would just be sure to put the correct finish over the galvanize to get the right appearance.    If you are going to expose the car to a lot of elements, the dip may be a good idea.

i was thinking of doing hot dip because of our varied climate in the north east of the uk,

Been there done that and it last a long time. The key is you must remove everything as the hot dip will melt it. This must only be done on a very good chassis.