best exercise to strengthen achilles tendon?

[shady]

im taking aome time to stretch and warm up (possibly not enough, mainly calf raises on steps) but for the last ten years or so everytime i get into running even a little bit its always my achilles tendon (usually the right one) that starts getting sore and if i dont cease running for a week or so it just carries on getting bad to the point ill be hobbled for a couple of months. im 48 so i understand im not as stretchy as i used to be but if the better informed people out there could advise me of the best stretches or exercises i should be putting time into id really appreciate it thanks. when i saw a physio he just said to continue the calf raises but obviously this isnt getting the job done.
id love to get back to a state where i can actually enjoy running again rather than dreading it

 

[napaulm]

heel drops. basically negatives. go up on both feet, come down on one,

 

[shady]

thanks napaulm ive started doing those along with a few that my girlfriend taught me. im still mobile so something must be working