Appart from digging them out and disposing of them to an appropriate site you are doing all that is possible, Da. One of the problems with hogweed is that the seeds can live for ten years or more in the soil! It is possibly new seedlings that you are dealing with.

Dawn, you have my utmost sympathy... have you tried smashing open the stems and then spraying? To be honest I'm not sure anything really works!

Thanks Bulb and Moon Grower. I think they've been around for years but only recently noticed they are thriving. As the roundup didnt work I took to the shears today as I dont want them to flower again, took me over 2 hours, as they are dotted around about an acre. I might get husband on the case with the spade to try and dig them out then. I guess they are very deep rooted. I could try smashing the crowns too and spraying them or tipping salt in. I dont want them spoiling the bluebells and grasses, blinkin things, they make docks,dandelions, buttercups and cow parley look quite insignificant.

http://invasiveplantsmi.org/hogweed/index.html

Read further down on this link till you come to how they erraticate it with a spray.

Dawn be very careful to wear gloves we dealing with them the sap can give very nasty burns.

Thanks Sixpence, I'll take a look.
Thanks MoonGrower for the warning.