The rails on the HW45 are 13mm. To fit a Red Dot you will need to find one that doesn't have integral fittings, but needs a pair of mounts as well. I have the same problem with my HW45.
Try Uttings www.uttings.co.uk. Hawke seem to do a Red Dot with mounting rings. Might be worth checking if they can sell without mounts. They also sell the BSA reversible mounts for a 30mm tube that will fit the HW45.
Hope this helps
Paul