A double-your-money challenge to raise £400,000 towards a hospital cancer care appeal has been launched.

The Forever Friends Appeal's Space for Cancer Care campaign is raising £650,000 for a new scanner at the Royal United Hospital.

The appeal's chairman John Cullum is also High Sheriff of Somerset and has launched the High Sheriff's Challenge to inspire people to raise as much money as possible.

Money up to £200,000 will be match-funded by former High Sheriff David Medlock, who has pledged another £200,000 from the Medlock Charitable Trust to double people's efforts.

The appeal is raising money for the latest and most hi-tech gamma-CT scanner, which helps in the diagnosis of certain types of cancers, as well as identifying cardiac and neurological diseases.

RUH deputy payroll manager Andrea Peters is one of the first to take up the challenge, and will be running the Bath Half Marathon and London Marathon to raise money.

She said: "I have been a supporter of the appeal for many years now, having had children in the old neo-natal intensive care unit, and have close friends and family who have been affected by cancer.

"I was so pleased when my application to run the London Marathon was accepted, and knowing that my funds raised will be matched £1 by £1 is the icing on the cake."

Anyone who sponsors her £10 will be able to nominate someone for her to run in memory of, and she will then run the marathon with a list of names on her back.

Mr Cullum said: "I have been amazed and impressed by what has been achieved in the voluntary sector, none more so than with Andrea, who is running the Bath Half Marathon and following a few weeks later with the London Marathon – that really is going the extra mile.

"I do hope people will take up the High Sheriff's Challenge. Not everyone will be able to run like Andrea, but there are lots of other things to do."

To sponsor Andrea Peters, visit www.justgiving.com/andrea-london2012.

For more information on the appeal visit www.foreverfriendsappeal.co.uk.