Thomas, Defender seats don't tip forwards, tho the back can be moved forward of vertical - the base clips in and out so you can get at the locker/fuel tank underneath. Your Mil. 109 will also have had - simpler and less comfy - lift out seat bases to access the fuel tanks, If I remember some actually had forward-aft adjustment ( the luxury !), on others the frame was bolted in place.
Word of warning - One of the events that hastened the removal of the Series LR from MOD service was a particularly horrific incident at the off-road driver training site at Tidworth. Two female Officer Cadets in a 109 FFR failed a hill climb and rolled the vehicle. The fuel tank cap seals were poor, the batteries in between the seats had a piece of loose metal ( spanner ?) floating about..... spark - whoosh. The two Officer Cadets died in the resulting fireball, their Permanent Staff Instructor who attempted to save them was VSI. If you do have a petrol series vehicle with under-seat fillers please remember that those seals are older than jehosephat and check that they work !