mickjoebill,
I used to work in the seismic industry and I’ve done a bit of bird towing (best 3 weeks of my flying career thus far). There are several short-range navigation systems available that will give you the required precision; almost all are being/have been phased out and replaced by some (often custom-made) form of differential GPS.
During the bird towing I was required to fly within 25m laterally of preplanned lines, at about 60 kts or 40 kts depending on the type of bird, as low as possible without wiping out the bird against a tree; after a bit of practice it was rare to be more than 5m off.
A display like a large (about 4” X 6”) PDA was mounted to the right of the instrument panel, showing the survey lines and the aircraft’s position relative to them. The position data points were captured with a precision of well under 1m in the x, y and z axes. The actual navigation system was differential GPS.
I don’t know of any off-the-shelf navigation system that will allow you to do the job you want but based on my experience I’d say one can be easily created. It will involve a differential GPS setup (a GPS base station on the ground sending correction signals via radio link to a “rover” GPS receiver in the aircraft) and a display for the pilot showing corrections to be made in X (forward-back), Y(left-right) and Z(up-down) axes. As your mission is photography the flights will be in day VFR conditions with good visibility and ground reference, so aircraft control will not be a factor. Since your camera mount will allow photography regardless of the aircraft heading you can just hover into the wind; with a known hover heading the position information can be easily converted from north-south, east-west, to the X and Y axes mentioned above.
Any land surveyor who is computer literate (and today that should be any land surveyor) can whip up a program to give you the above display in half an hour or so, and they are also used to working with mobile DGPS systems. I’d suggest getting into contact with a land survey company (come to think of it, a company called Fugro do a lot of aerial surveying worldwide, I’m sure they could get you pointing in the right direction) or the exploration department of a mining company and explaining what precisely you’re after.
Edit to add:
I've just been on the Trimble website (
www.trimble.com) and it would seem they have handheld receivers/computers which could fit your bill even in basic (non-differential) mode with the right antennas. Try giving their UK representative(s) a call.