I put the door check straps in today. It was a bit of an ordeal so I thought I would write a post in case I ever need to do it again or if someone else ever needs some advice. Firstly, original ones of these are a nightmare. Pretty much all of mine were broken and the ones that weren’t broke when we tried to refit them. I tried to find some good originals second hand or NOS but no luck. In the end I ordered a new set of repros from the Cortina Club which were painted up to match the car. When fitting I got a few scratches on the first one unfortunately as I worked out how best to squeeze them in but after that everything was good. I also had my locking straps re-plated and used new hardware so everything looks as good as new now. Instructions on the photos.
Step 1 – Place the strap in position and put a chock to stop the hook from jumping out. Tape up the end of the strap as you will need to tap it into place. 
Step 2 – use a flat blade or similar to push the strap down so that the front can be pushed into the groove. I found a chisel worked best to push on the tangs as a screwdriver was too narrow and there is a lot of pressure required. 
Step 3 – with the pressure on gently tap the strap until it falls into position. It may not completely seat but as long as you can get the retaining strap on then you can tap it home later without it jumping out. 
Greg's Cortina Mk1 GT Build 