BoilermateSP sporadic hot water and does not drain

I have a BoilerMate SP which is just over 20 years old. I've noticed now (in winter) that it struggles to provide continuous hot water. It will do a few minutes and then goes cold. After maybe 20 minutes the water is warm again. I think something is disrupting its ability to shift heat around. I live in an area with hard water and suspect it could have a lot of scale inside it.

I thought I'd try and drain it myself. It's been drained once before maybe 10 years ago to replace the immersion heater. I attached a clear PVC pipe and I've opened the drain cock but nothing is coming out. Does anyone have any tips on next steps? Thanks in advance.