When Im driving at a steady pace such as in traffic or on the motorway at slowish speeds, or have taken my foot off the accelerator, Ive noticed that on odd occasions between 3k and 4k rpms the tachy blips between either of the two, as though it cant quite work out where it should be. Its been becoming more frequent lately, does anyone know what this could be?

There is no audible difference in the engine, the car doesnt kick back or anything either. If I wasnt catching the tachy out the bottom of my vision I wouldnt have noticed anything at all.

Electronic Spedo sensor on the gearbox?  My old Ibiza used to do that until i replaced it.  

Knock sensor will have nothing to do with it, a knock sensor is there to detect knock and allow the ecu to retard the timing if knock is detected to prevent detonation.

O2 sensor failing will give you violent jerking when acelerating at certain rev ranges, or generally pish running

thermostat will see your temperature behaving oddly, or the heaters never getting warm when driving if its stuck open

What drives the rev counter on them - is it a pickup on the crankshaft or is it impulses from the coil. could be a faulty sensor. The speedo will prob be driven by pulses from the ecu from a sensor in the gearbox

The opening up at 4200rpm will prob be the start of the powerband (As its a 16V motor)

take your plugs out and inspect them to see if it looks like its running rich etc

You only need a plug spanner, and a wrench.

Take a photo and show it to mark.

Have you asked on the mx5oc forums?

I did have the same prob in one of my minis, it was a loose wire but it sounds like you got something different going on. If the engine isnt doing anything odd I wouldnt worry too much - its 17yrs old like you say.