Trains are timetabled from London Paddington (where the Heathrow Express also comes in). Some are direct. Some you have to change at Newport, Gwent. About 3 hours.

Looking at http://herefordbus.info/home.php, clicking on the map for Hereford North and chosing route 445, there seems to be one bus a day leaving Hereford at 4 pm and arriving Hay at 445. Alternatively your hotel may be willing to fetch you or arrange a taxi.

Multimap shows the bus and rail stations in Hereford as being about 200 yeards apart.

You'll need to take Stagecoach Bus 39 from Hereford to Hay. There are trains from London Paddington to Hereford, but you may need to change at Worcester or Malvern.

Train (Mon-Fri): http://www.firstgreatwestern.co.uk/link/servicepdfs/London-Hereford(Weekday(rev)).pdf

Train (Sat-Sun): http://www.firstgreatwestern.co.uk/link/servicepdfs/London-Hereford(Weekend(rev)).pdf

Bus: www.stagecoachbus.com/timetables/Serv39.pdf

Well you dont say if your going to be in a car or not ...... if not then the advice given already is satisfactory

If by car :http://maps.google.com/maps?f=d&source=s_d&saddr=london+heathrow&daddr=Dorstone,+Hereford,+County+of+Herefordshire+HR3+6BN,+UK&hl=en&mra=ls&sll=52.066699,-3.009009&sspn=0.006147,0.013819&ie=UTF8&z=8

http://www.thegoldenvalley.co.uk