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The coolant level in my header tank keeps dropping. I fill it up to about an inch from the filler neck and after about 200 - 300 miles it's dropped to the point of the coolant light coming on. There's never any coolant on the floor or any signs of a leak. The oil is clean and no mayo in the system to suggest any head gasket problems. The engine runs fine and the temp gauge sits at halfway whatever the driving conditions. I'm thinking of trying a new pressure cap in case it's not holding the correct pressure in the system. As far as I can see the only place it can possibly come out is down the over flow pipe and out by the front wheel, but there's no drops on the floor where I park.

You could maybe rig up little bottle in the engine bay and put the overflow pipe in it,
if it's blowing coolant out you'll see it

All my 7s have run with the coolant a little over 2" below the cap on the header tank. All bar one of them would settle there, & run for months, & thousands of kilometres at that level with no problem.

With one that ran a little warmer, I fitted the overflow, rebreathing bottle from a Toyota Cressida. These are square & fit nicely in the gap beside the radiator. This kept the coolant right up to full.

I fitted the same thing on my current car, when I fitted the new engine, the air conditioning system, & a high output alternator, as insurance against overheating. It works perfectly. I use a 15 PSI rebreathing cap.

Hasbeen

If you over fill the tank it will leak out the overflow when it expands with heat but your low coolant light shouldn't come on as it will stabilize at proper level (you may not experience any drips on garage floor as it would drip while under way).

It could leak out the rectangular hole on side of block if your water pump seal has failed. I don't know if you would see any drips on garage floor.

It could be drawn into the combustion chamber if the head gasket has failed and expelled as water vapour out the exhaust. This is usually evident by excessive white vapour (smoke) at the tail pipe. You may get a little initially when the engine/exhaust pipe is cold which is normal as it's from the water vapour in the air.

We can't get 15 psi caps locally (newer cars take 16 psi or higher) here and have to be ordered in from regular TR7 suppliers. Although only 1 psi higher I'm loath to put more load on the system than is required on an old system.

Further inspection today has revealed a lovely brown rusty coloured river has run along the top of the block just below the cylinder head, and down over the bell housing. I'm thinking a leaky head gasket. Along with a suspected sump gasket leak I'm considering pulling the engine out for some TLC. Probably easier in the long run than fiddling about with it in the car.
I'll try Garry's idea of sticking the overflow pipe in a bottle on my next decent trip just to eliminate that possibility. In any case the car is coming off the road for a while in a couple of months so it will be sorted in the fullness of time. The garage is going to be taken over by Project Banana.