A key found at the home of murder suspect John William Cooper fitted a lock on a bedroom door at a property owned by victim Richard Thomas, a jury heard this afternoon.

Mr Thomas, 58, and his sister Helen, 56, were shot dead at their home at Scoveston Park on December 22, 1985.

Mr Thomas also owned nearby Norton Farm where, a witness has told Swansea crown court, Cooper sometimes worked as a casual labourer.

In 1998 Cooper was jailed for 16 years after being found guilty of 30 burglaries and a robbery.

During that investigation police dug up the garden of his then home in St Mary's Park, Jordanston, and found several keys.

Robert John Phillips said he bought Norton Farm at auction in 1986 from the estate of the late Mr Thomas.

He said he extensively renovated the property but did not change all the locks.

He said in 1998 police came to Norton Farm and tested a large number of keys on locks that had been in use in 1986 and not replaced during the renovations.

Mr Phillips said he had never had a key to a bedroom door--but one the police had with them fitted the lock.

Master locksmith John Crummock said he tested the key and the lock and he was "one hundred per cent certain" the key had been used in the lock for many, many years.

Mr Crummock said it was not the key manufactured for use with that particular lock, but it still worked.

Because it was slightly smaller than the original it did not fit perfectly and as a result had caused tell-tale damage to the lock. Wear on the inside of the lock and the key left him in no doubt it had been used over a long period of time.

"It fits like a glove," he added.

Cross examined by Mark Evans QC, the barrister representing Cooper, he agreed the lock was cheap, basic and mass produced, and that different keys could have made it operate.

But although the key in question was the wrong key, it had been used constantly in the lock of the bedroom door, said Mr Crummock.

The barrister maintained it was nothing more than a rusty old key that happened to operate what was a very common type of lock.

Cooper, aged 66, now of Spring Gardens, Letterston, denies the murders, and the murders of Oxfordshire couple Peter and Gwenda Dixon, aged 51 and 52. They were tied up and shot while enjoying a last walk after a two week holiday in Pembrokeshire in June, 1989.

Nine years later Cooper was jailed for the burglaries and a violent robbery.

Items found by police during Operation Huntsman, which led to his arrest, were subjected to new forensic analysis between 2007 and 2009 as a result of which Cooper was charged with four murders, rape, indecent assault and five attempted robberies.

Cooper denies raping a 16 year old girl in a field close to the Mount Estate, Milford Haven, on March 6, 1996. He also denies sexually assaulting her friend and trying to rob five teenagers of cash.

The trial continues.