If your low coolant light is coming on (and you talk about "topping up") it sounds like coolant is disappearing somewhere, but no evidence of an external leak? A cooling system pressure test could narrow things down.

If you can obtain an infra-red temperature sensor (or find a helpful repair shop, and safely get the car to it) this will confirm whether your car's gauge is accurate and you do have an overheating situation.

An internal coolant leak and excessive engine temperature when under load, could indicate a leaking head gasket. Try to get a cylinder compression test, or a "leakdown" test. Most shops have the equipment to do this.
--
Bob: son's XC70, dtr's '94-940, my 81GL, 83-DL, 89-745(V8) and 98-S90. Also 77-MGB and some old motorcycles.