Derek,
If it is the one in the picture, it looks to be in fine condition. I have seen them with original factory box in excellent condition sell for $1,500 here in the USA. On the low end for a gun in poor condition it could bring $150.
Very technically, the gun is a Harrington and Richardson. H&R is a later company. There were 3 Models, each has 3 Types and came in a variety of gauges and calibers. Serial numbers have been observed from 1 to 54,000. Value will depend on Model, Type, gauge/caliber and condition. Some were smooth bore, others were rifled. The smooth bores were made from 1921 to 1934.
In the USA, the smooth bore guns were placed into a special category by the NFA (National Firearms Act) of 1934. Each gun had to be registered with the federal government and was issued it's own tax stamp. The transfer can only take place using the services of a FFL III licensee. It makes collecting / owning these guns a bit trouble some. Any smooth bore gun that was not registered is illegal and illegal to possess.
Pete