With my B310 Sunny's, I always back off the clutch stop bolt on the pedal box, then pull the clutch pedal up, which will move the hook on the top of the pedal. You should be able to slip the cable end over the hook. you then adjust the stop so the clutch pedal is the same height as the brake pedal & adjust the clutch cable at the firewall. You can also change the clutch cable to one from a 120Y or a Sunny, which have the larger body needed to fill the hole in the bell housing. The thickness of the washer could be just enough to make it impossible to get the cable on the clutch fork.

Have you swapped from an A12 to an A14 or A15? The larger engines have a thicker flywheel, so the distance between the gearbox face & the clutch fork when the throwout bearing touches the pressure plate forks is greater. I always used the A12 flywheel, as it's like fitting a lightened flywheel. Are you're using the plate between the engine & the gearbox? Removing this will move the gearbox forward & should help with fitting the cable.

Thanks guys. Everything is definitely loosened off. Will tackle again this weekend.

this can be tricky.

Don't connect the pedal end to the cable. Slip a small cord over the end of the fork in the gearbox that pokes out the gearbox to pull up the slack on the thrust bearing spring in the gearbox then slip the cable over the fork end... tie the cord off to the front of the car somewhere to maintain the leverage (to help give you some slack to get the pedal end on)
After you have the gearbox end on then insert the cable through the firewall and you may need to remove the pedal bolt, slip the cable over and re insert the pedal bolt/pin ... all depending on the slack you can get on the gearbox end with that piece of cord.

Remove the cord (cut it off) and you should be right.

Don't think so. common problem is loosening the cable (the large nuts at the firewall) and make sure the clutch pedal is all the way up

changing the engine problem can be
* thicker flywheel
* engine plate/no engine plate

What have I done wrong here....

Engine and gearbox removed. Hence, clutch cable removed also. New engine fitted with same gearbox and same cable. Gearbox is a 'large hole' so uses a washer to locate the cable in the box housing. For the life of us I cant get the cable to reach through the hole and over the clutch fork hook. There is no reason why it should reach. Yes its all undone and made as long as possible etc etc.... Has the clutch being bolted onto a new block changed this somehow? What am I missing?