The Friends of Monk Bretton Priory have again had to deal with the fallout of a break-in, with keyholder Victoria Agnew - who lives in the cottage in the grounds - saying the vandals had damaged stonework and a window in the late 13th century administrative building.
The former Cluniac monastery stayed locked up over the weekend after the break-in last Wednesday.
Because of its Grade I listing, repairs have to be carried out to exact specifications and can take months.
Work is also costly - as is the introduction of measures to stop further incidents.
“We are having to put more procedures in place,” said Victoria.
“I’m on my own so I’m taking up quite a bit of police time. It is intimidating, especially when they come back while I’m waiting for the police.
“We have got a lot of events planned for the summer and it’s disheartening when something like this happens.”
Group members say the site’s gatehouse, one of the newer parts having been dated back to the 15th century, has previously had to be locked up for safety reasons.
The administrative building was targeted by vandals in March, leading to a fundraising campaign to restore its roof and windows.
Drug paraphernalia has also been found, according to former keyholder Darrell Haywood.
He said it looked as if the yobs had used an axe or angle grinder to gain access to the administrative building.
Darrell said: “When I was the keyholder, I noticed that at 3pm - you could set your clock by it - it seemed as if these people saw it as their time to go in.
“They know they’re doing wrong and they shouldn’t be there, but it seems like they want a confrontation, they want you to phone the police.
“It’s a game to them. They don’t know the value and the history of the property.
“I’ve phoned the police time and time again. Victoria is on her own and it is frightening.
“If they were that way out, they could cause a fire and it would end up ruined.”
Darrell added that English Heritage was planning to install new CCTV which would ‘hopefully be able to identify these vandals’.
“They need to be prosecuted - fine them, take them to court.
“If it gets out of hand, that’s a building that’s 900 years old and is going to be lost.”
Members are still hopeful that a schedule of events over the summer can go ahead, as well as future plans to create a Cluniac-inspired trail alongside the neighbouring Mill of the Black Monks pub.
Cluniac Federation visitors have said the priory is one of the clearest and best-preserved sites they’ve visited in Europe.
Victoria added: “It’s not a nice thing to happen, but it kind of has brought the community together - rather than letting them keep getting away with it, let’s get the building used rather than it being an empty shell.
“We’ve had 40 volunteers take part in our fundraising since the first break-in.
“That shows people are of the same mindset - let’s get something positive out of it, it’s part of Barnsley’s history and we can’t leave it to rack and ruin.”
Barnsley Central MP Dan Jarvis, a supporter of the group’s work, said: “I am utterly disgusted by reports of antisocial behaviour and vandalism at Monk Bretton Priory, which is putting this fantastic cultural and historical asset at risk.
“I would urge anyone with any information about those responsible to report it to South Yorkshire Police.
“I’d like to thank the amazing volunteers from the Friends of Monk Bretton Priory who are working incredibly hard to make this a safe, enjoyable place for families to visit.
“Their hard work and dedication must not be undermined by the loutish behaviour of an antisocial minority.”