I fly down drom athens,greece to kl next sunday and am thinking of getting a 60 day thai visa whilsty still in athens

Though i'll be booking a flight from kl to bangkok in the coming days i dont have a flight booked as yet and so no definite day of arrival in bangkok

I can show a definite departure date from thailand however which will be about 6 weeks from now so will i need to show an arrival date in thailand for my visa application to be processed and indeed once this flight has been booked will said visa still be issued even though my initial arrival point will be malaysia

Also,what length of stay visa will be issued upon arrival at kl airport(i am a uk passport holder)

on the latter point i've just found its 90 days for uk citizens

What Thai Consulate in Athens will require is up to them - they may not need any such information. Another option is obtaining a tourist visa in KL if you have time there as very easy - apply in morning and pick up next workday afternoon.