Quick update:
Didn't actually do it in the end. There were no local laces to us to hire bikes from (that were open) and as it was a short family break I didn't want to spend a day pleasing myself.
I wouldn't have fancied road riding, the Cretan driving is pretty poor and they genuinely believe that when they overtake on blind bends, 50% of the outcome depends on whether God or the Devil is looking out for them
We did a tour up into the mountains and there are some big buggers there! the largest is at something like 9000 feet although the whole of that area is extremely mountainous.
If we go back for longer, I would seriously consider taking a MTB as there looks to be some amazing riding there albeit most of it requiring full body armour.
Longers, if you're considering it I can strongly recommend it. I'd never fancied Greece before, it's only because a colleauge at work really sold it to me, he first visited 27 Years ago and has returned every Year since!
The people are incredibly friendly and it's a lovely, relaxed place (if you stay away from the roads

)
We did pick a good time of Year, a lot of it was shut down so it was fairly quiet (we don't like crowds.)
Saying that, I think maybe a couple of weeks earlier would've been good, the weather was breaking down when we were there. We had a few rainy days and yesterday, it was only 3degC warmer there than at home, although one day it hit over 30 and was generally either warm or hot.
Did lots of swimming although the sea temperature seemed to be dropping fast as after a couple of Hours I was that cold I'd had enough, and I do a fair bit of swimming in the sea in the UK.
Thanks again for the advice all, when we return it will be for a little longer and I will hire a car so I can explore some of the suggestions next time.
Just to add, I would definately take a tool kit and wouldn't ride alone in the mountains. They are extremely remote, there is no mountain rescue and no police helicopters to come out for you.
The Griffin vultures (we saw loads) would pick your bones clean and then the resident
Lamergeirs dispose of the bones so the only evidence of any nasty accident would be your broken bike...