Hi Neil,
Tried and successfully reset the Nuvi and also cleared nonvol. However this made no difference. Screen is still very slow and laggy.
I have two other options and would appreciate your thoughts ...
First is to use windows explorer and move the map folder from the Nuvi onto the sd card, thus freeing up about 3.5gb and giving the unit more space to operate, or secondly I've found the 4.0 firmware from an old Catymag post which I have made into a rgn file ready to reflash. However I'm a bit reluctant to flash the firmware after reading the post Boki has shared as I might make things worse.

Rusty