Thanks for the replies. The deck itself is level and straight, so I think that I may be able to fill in the gaps with some soil and then block in the part on concrete with thin strips. I don't suppose anything will stop vermin if they wanted to move in.
On the subject of cost, most of the quotes I got were using feet and yards and were working out around €100 sq.yd. fitted for soft wood, double that and more for hard wood. So maybe a little extra per sq. metre. ABS on the Naas road have 16*16 supply only for €699, no handrails.