I've got a massive 300+ mile (600+ round trip) 5hr journey planned for this week. 

I'm after tips for driving extremely long distances  / driving in Cornwall, I've heard roads are extremely narrow in places. Are there any wildlife havens such as in the new forest  I should know about? I don't think it's still tourist season? Or is it?

Any tips welcomely received 

Loads of country lanes that involve finding passing points or gateways that aren't blockaded by big rocks or chains.

The A38 is twisty and rubbish but the A30 is ok although exposed over Bodmin Moor.

You have the seal sanctuary in Gweek & Newquay Zoo.

If you come down the A30 you must give the Jamaica Inn a visit.

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Driving in Cornwall is either a love or hate experience lol

One of my Mum's best friends used to love visiting the Jamaica Inn

Never been myself

Many many thanks for the tips so far, they are welcomely appreciated.  I'm getting nervous about the trip already and have visions of having to reverse for miles to find a passing point lol

What's the correct etiquette for blind corners of country lanes? Is it a polite beep before I take it or just take it slow and steady and just hope for not a speeding motorist coming the other way?

Country lanes = take it slow and don't rush. Be prepared to reverse up the lane a long way if need be. 

But to be honest, 300 miles it not a particularly long drive. If you find yourself getting frustrated and/or tired, simply take a break.

Edit: I tend to have a break every 2-3 hours on long drives, regardless of mileage covered in that time. (Sometimes I've covered 300 miles but that point, other times it;s only 50 :-(  )

Not sure where you live Laurie but aren't there tight single track country lanes near you?

There's a lot near me, I used to hate them but got used to them pretty quickly tbf.  The main thing with them is keeping concentration as it only takes a split second of glancing away and a car appears in front of you, very different to motorways where you can look around or change a CD etc at the same time!

Passing places are just common sense really, keep note of any you pass just in case you have to reverse, and if you've just come round a blind bend usually the other person will have the sense to reverse where they have the visibility.  Most of the time you'll meet people far enough away that you can pull forward into an upcoming passing place.

The part you'll hate having a new car though, is all the over hanging branches and bushes that can't always be avoided!  

I do beep on very tight corners if there aren't any houses on the bends, otherwise I just take it as wide and slow as possible and hope for the best!

The driving conditions you talk about describe over 75% of the roads in my area - to get practically anywhere you need to spend at least a few miles on country roads with lethal bends and blind summits. 

But, Tom has covered all the points so there's nothing more for me to add!  Just keep your eyes on the road (I mean 100% of the time!) and use your horn where suitable.  I also beep on particularly bad summits and bridges that you can't see over.  After having the O/S mirror on the Primera whacked off by a speeding Astra over a blind summit I've always been extra cautious, as a few inches closer and it could have been the entire O/S, or God forbid, a head-on collision.