A heritage chief today urged residents to challenge people they saw digging up Yorkshire stone paving and report them to Bradford Council.

The Council's heritage manager, Steve Bateman, spoke out today after a retired university computer lecturer spent a night on security duty outside her Thornton home to protect her 200-year-old Yorkshire slabs from thieves.

Conservative ward councillor Clive Richardson - also a police authority member - said theft of the valuable flags was rife and the residents and police must work together to stop it.

Lesley Semmens was bleary-eyed but jubilant after spending a night guarding flagstones. Slabs had been lifted from a courtyard outside her historic cottage in Field Court, Thornton, on Monday night.

The retired university computer lecturer believes thieves had lifted them ready to steal but had been disturbed and left them stacked up. So to make sure they stayed outside her home she spent all night watching them.

The 54-year-old said after her round-the-clock vigil: "I bought this house because I loved the area. It's beautiful and I'm not going to let them spoil it."

Dr Semmens called the police who told her they could not get involved without a suspect.

Then she contacted Bradford Council's highways department who sent their on-call expert round.

And even though the road is unadopted and not legally under their care they brought cones and marker spray to use on the flags.

Dr Semmens estimates the eight flags are about the same age as the cottage and probably would cost about £400 to have laid.

Robbers have already stolen York-shire stone slabs which had been legally lifted by workmen in the High Street in the village.

As the workman had their lunchbreak the thieves loaded the stones on to a wagon and drove off. Dr Semmens said: " The man from the Council was excellent in helping me.

"I watched lots of films and drank lots of coffee and kept checking on the stones.

"It would have taken about half an hour to load them on to a wagon so I was ready for them. I daren't go to sleep.

"But even if I did, I think I would have heard them."

She now plans for a friend to lay the flags back down embedded in concrete to make sure they are not taken again.

Heritage manager Mr Bateman said: "Any loss of traditional material is regrettable and it is happening in key areas and the city centre."

He said it was difficult to tell whether people were digging streets up legally because there was genuine work to do. But anyone who was unsure should challenge them and say they were contacting the Council.