When Oxford businessman Captain J J Cullimore Allen received three brand new five pound notes in 1957, his first instinct might have been to go and spend them.

Instead, Capt Allen decided to hold on to them. They have since made their way into the hands of a mystery collector and are set to be sold next month for more than £800.

The reason the bills have become so valuable is because they were from the first series of coloured £5 notes to be issued by the Bank of England in 1957.

They were also the first three coloured notes issued by the Westminster Bank after the new notes came in to replace the old black and white ones.

Barnaby Faull, bank note expert at auctioneers Spink, said: "What is interesting about these notes is the letter that accompanies them.

"It is from the Westminster Bank manager in Oxford, a Mr Foster, telling him they are the first of the new bills to be issued by the bank.

"That makes them very special.

"You don't normally get that sort of thing with notes like this.

"All I can imagine is that Capt Allen was an important customer of the bank for them to do this sort of thing."

Capt Allen and his family owned John Allen and Sons Ltd, in Cowley, which was one of the leading manufacturers of steam ploughs in the country.

The site of the firm was redeveloped in the 1980s as the John Allen Centre retail park.

Mr Faull said he expected the notes to fetch as much as £850 when they go for auction at Spink, in Bloomsbury, London, on June 19.

He said: "Good stuff will always sell well but these are of a particular historical value so I imagine there will be quite a bit of interest from collectors.

"It is because of the letter that we have decided to sell them as one lot. They are quite an oddity and things like this don't turn up very often."

Capt Allen was sent the notes in February 1957 by H S Foster, the manager of the former Westminster Bank in Cowley Road, who wrote: "I have pleasure in handing you herewith three of the new issue of £5 Bank of England notes."

Back in February 1957, £5 was about a week's wages for the average female Oxford worker.

The average house in the city cost just £2,330, a gallon of petrol cost six shillings and a halfpenny - about 30p - and a pint of beer set you back one shilling and threepence halfpenny - all of 6p.

For a free valuation of old currency, contact Mr Faull on 020 7563 4000.