I was always taught that when testing new and regular tested fittings a 1hr duration was an acceptable period but when testing fittings that have not been tested a full 3hr duration had to be undertaken. OIs this still the case and where can I get confirmation of this in BS5266 ?

That went out with the ark!

Currently BS5266 only requires monthly function testing and annual full rated duration testing, which for 99.9% of fittings sold and installed after the last 20 years are 3 hour rated. This has been the case since 2004.