Power Meters / Powermeters (SRM, Powertap, Quarq, Ergomo, Vector, Stages, power2max, P2M, 4iii, InPower, Cinch)

ok a bit more googling suggests doing a 3minute aerobic test resting a bit then doing a 20 minute high percentage of maximum minute power test - does that sound like a useful use of an evening...?

Its the first thing you should test. But I'd still ask what distance you are focused on.
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The 20 minutes Test

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The 20-minute Critical Power test is conducted as follows:

•On a relatively flat, UNINTERRUPTED section of road, warm-up easy for 15-20
minutes

•Over the next 5 minutes, do five (5) hard 30-second efforts, followed by 30
seconds of soft-pedaling. The purpose of this drill is to open up the blood and
oxygen flow and to increase the heart rate prior to the 20-minute effort, so
don’t go too hard. Push a wattage you think you can sustain for 10 to 20
minutes.

•Pedal easy for 5-minutes and prepare yourself mentally for the 20-minute
test, as it’s going to hurt!

•Start a new interval on your power meter and immediately begin the test.
Start the 20-minute test by selecting a wattage you think you can sustain for
the full-20 minutes. The cardinal rule of time trialing applies here: don’t
start out too hard. Keep in mind that the best cyclists in the world can only
sustain 400-500 watts over a 1-hour period of time, so if you find yourself
starting out at 500 watts, you know you are likely going much too hard. It’s
best to start out easy for the first two minutes, and then build progressively
to a wattage level you think can sustain.

•Hold that level for the first 15 minutes, and then give it your best effort
during the final five minutes. (If you find yourself fading in the last five
minutes instead of holding steady or building, then you may have gone out too
hard. Keep this in mind for your next test).

•After 20 minutes, immediately start a new interval to save the precious
power data you just worked so hard for! Cool down completely and head home.

Your Functional Threshold Power will be 95% of you average power during the
20 minute effort

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