Wow , I have the complete opposite problem which I can't figure out , the colder it is the better it starts (as in almost instantly , so I really feel it's not the glow plugs as that normally affects cold starting) , the warmer it is the more likely I will have to crank it more than once to get it to start and it will blow blue smoke (unburnt diesel) when this has happened , on the first crank it just won't fire , or it will take a lot longer before firing but now I know when it doesn't fire in the first couple of engine rotations to stop cranking and go again and it almost always instantly starts the second time.
So basically it starts perfectly during all winter and then in summer I regularly experience the starting issue.
I have asked on here a few times without really getting any closer to the problem , one of the suggestions was to clean the MAF which I have done but it has made no difference.
I do have glow plugs I purchased quite a while ago but am a bit reluctant to change them due to the mileage (230,000kms) , I'm a bit worried about seized glow plugs.