631 was the first of a batch of 10 Sunbeam S7A trolleybuses which entered service in Huddersfield in November/December 1959 to replace some of the ageing rebuilt Karrier E6 trolleybuses. These were Huddersfield’s last new trolleybuses and the last three-axle trolleybuses built for service in the United Kingdom. They followed the style of the previous Huddersfield trolleybuses (3-axle, double-deck, 7' 6" wide) to fit in with the rest of the fleet, despite the regulations having been relaxed a few years earlier which would have enabled these trolleybuses to be built as two-axle and 8-foot wide. They were not particularly well-liked by the Huddersfield crews, who tried to have alternative vehicles allocated to them.
Life during 631s nine years in service was uneventful. However, when the trolleybus system in Huddersfield was abandoned in July 1968 the Huddersfield Trolleybus Preservation Society (HTPS) secured 631 for preservation. She was used for two successful enthusiast tours of other systems before she was 'retired' to museum work. On 27 October 1968 631 toured the Reading system with Belfast 168 and she ran another tour in Bournemouth on 24 November 1968.
631 was transferred to the site which became the Trolleybus Museum at Sandtoft in 1969. In 1974 the external paintwork was stripped and repainted.
Following the merger of West Yorkshire Transport Circle (one of the groups which grew out of the HTPS) with the British Trolleybus Society, 631 became part of the BTS fleet.
The power supply at Sandtoft up until April 1992 had been provided by a diesel generator. From 19 April 1992 the power supply for trolleybus operations was switched to mains electricity and Huddersfield 631 was the first vehicle to operate using it.
In 2012 631 was decorated in true Huddersfield style to mark the Queen’s Diamond Jubilee.
On 10 January 2013 631 was towed to Swindon where she was externally repainted back into Huddersfield livery and a partial internal repaint was carried out. The seating was re-upholstered too. On 3 May 2014 631 was returned to Sandtoft and was recommissioned into service later that year.
Arguably 631 has been Sandtoft's most reliable service trolleybus over the years and is still in good condition.