Thanks to everyone for their input on this. Lots of good ideas, some of which I'm just about capable of carrying out...
One of the things I was doing wrong, that I now realise from the forum responses, was that I was just measuring the o/p from the IR sensor using a simple voltmeter. Of course, it seems really obvious now, but I really should have been using my scope. I'll have a go with that over the weekend and see what that shows up.
Anyhow, I am still VERY interested in being able to build a circuit that can filter out the 36KHz frequency being emitted by the IR diode. So far no-one has posted any circuit ideas, or component values that might help me to build this. Yes, I know I can build my detector in other ways, but this stuff really fascinates me, so I am using this 'project' as a learning vehicle.
I'd really love to be able to build a receiver circuit that I could connect my scope to and see a reconstructed waveform, so that if I were sending a 36KHz IR signal, which was turned on and off regularly and which generated a waveform similar to the attached image
https://www.electro-tech-online.com/attachments/crotracea-jpg.29674/ then I'd be able to 'see' a very similar waveform appear on my scope attached to the receiver circuit.
Unless I have completely misunderstood the requirements of the receiver, then I reckon all I need is a suitable circuit that can filter out all/most of the other spurious IR signals, and just concentrate on the 36KHz one.
Like I said earlier in the thread, I think it would consist of a simple amplifier followed by a filter (or vice versa).
However, I really don't know how to build the filter, i.e. what components and values to use so that it only passes 36KHz.
Any help in this area would be really appreciated.