DH needs to get into London once/ twice a week and ideally would like a direct train no more than an hour and half away (would do more if the perfect locations). Would like to be westish/north of London so we can get to family easily.
Need somewhere semi-rural, maybe costal in a pretty area where we can rent a 3/4 bed house for less than £1200pcm.
Another vote for North Herts I am in Welwyn Garden City , nice town with lots of lovely countryside on our doorstep. Trains into Kings Cross 20 mins but you would only get a 3 bed semi for the money you mentioned
If you want to go westish, how about heading out to Winchester? You can get a 3/4 bed house for what you are prepared to pay and you can get the coast quite easily from there too - approximately 30 minutes drive. It is within 1.5 hr commute too. Lovely place - also has the new forest within a short drive.
If you want to go North of London, there are some very pretty villages around Chelmsford or a little bit further north there is Saffron Walden which is very picturesque. You aren't too far from the coast there either. Probably Chelmsford is within 1/2 hour of the coast too.