A £20,000 reward has been offered for information to help catch the killer of a grandmother who was bludgeoned to death as she house-sat for friends at a mansion.

Artist Valerie Graves, who had lived in Scotland until shortly before her death, was found at the secluded £1.6 million property in Bosham, West Sussex, on December 30 last year.

Ms Graves had been house-sitting with her mother, sister and her sister's partner while the owners were away on holiday. All three were also in the house at the time of the attack.

On the nine-month anniversary of the murder, Crimestoppers is offering a reward of up to £10,000 for information leading to the arrest and conviction of her killer.

Sussex Police offered a £10,000 reward for information in March.

Detectives believe the 55-year-old was attacked in her bed during the night by an intruder using a hammer. She died as a result of severe head injuries.

Ms Graves, who was well-liked with two grown-up children and two grandchildren, had graduated in textile design from Heriot-Watt University. She had lived in Scotland for around a decade.

After leaving the university, she started her own crafts studio at Harestanes, near Jedburgh, Roxburghshire, and lived in a cottage in the village of Maxton.

Staff at Heriot-Watt described her as someone who had a wide range of different artistic interests. During her time in Jedburgh, Ms Graves gained the respect of the artistic community in the Borders area, and had won awards for her artwork.

She returned to her home area of West Sussex to care for elderly mother.

Detective Superintendent Nick May, from Surrey and Sussex Major Crime Team, said: "We are appealing to anyone who has any information or knows the person who has done this.

"You may have your own concerns or suspicions about this person already, but for your own reasons have not yet come forward."

Mr May added: "Let me reassure you that we can support you whatever your concerns, but this person must be caught."

Angela Hollinshead, Crimestoppers' area manager for Sussex, said: "Valerie's murder was extremely brutal and her family has been devastated by her death.

"It is hoped that, by increasing the reward, this may assist in convincing someone to come forward and provide us with the critical information that may solve this crime.

"I urge anyone with information to contact Crimestoppers immediately without fear of their identity being revealed.

"No-one who provides information to Crimestoppers need ever be concerned about giving statements to police or having to go to court."

Ms Graves was last seen when she went to bed on December 29 and was found dead by her relatives at 10am in her bedroom.

A 22-year-old man who was held in connection with the murder in January was later freed without charge.

Sussex Police has set up a ­dedicated website in 80 languages on the case.