A single form linked to from all the pages under Custom in the menu.

Dropdown list of all the custom products which they select one or multiple products they want to get a quote for?

No dependent fields?

Finish options - just a link od these need to form fields - single or mult select?

What are the other fields? E.g. File upload, space dimensions etc?

Email address to submit form to.

Taken from the stairwells page on current site, but not sure if these are relevant for all custom products:

1.Pendant choice
2.Floor to ceiling heights
3.Size of void
4.Lowest drop height
5.Highest drop height
6.Ceiling specification - slanted roof, solid structure, light wells

What type of fields.

Measurement units preferred.

The other way of doing it:
If you want people to only get quotes for one product, we could have the data from the previous page populate the first field.