Can someone recommend a good plugin power meter i can use to start tracking down which of many electrical devices are responsible for my ~300W base load ?
Are there any that (for example with custom firmware) can be made to play nice and act as a node for use with the emonbase ?
Thanks.
Turn them off? 
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I have a small meter with an LCD display - it will give you a rough figure something like this. You are only looking for order of magnitude.
I also use this type of plug Amazon.co.uk flashed with Tasmota using TuyaConvert so they report via MQTT and easily imported into emoncms if needed.
However, be really careful the plugs have the right chipset else they cannot be converted. I ordered some off amazon but could not flash them as they were the wrong chipset.
Other alternative is to use a Sonoff POW2 and wire it into an extension lead. Again I flash with Tasmota.
[edit]
If the plug appears on this template page then it is probably flashable. UK Devices
Lol. That’s one solution. I have a fair amount of kit that is supposed to run 24x7 (like network gear). I also know from experience that power cycling (or rather switching back on) tends to be when kit dies. it’ll be nice to see which ones are the power vampires though and see if its worth replacing them. It is interesting start seeing what is going on - i have something (fridge or freezer i guess) that kicks in periodically for 20-30mins that pulls ~ 350W. it’ll be nice to see which it is.
Thanks for the recommendations. I’ll spend some time studying the links you provided and give it a go.
Of course to fit the power meter you need to switch them off 
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I have a small HP Micro Server that I have just discovered pulls 40W continuously. I used it as an ESXi platform. I’ve moved everything to an old laptop running Proxmox which draws about 8W for the same job with the added benefit of a builtin UPS. Plus Proxmox is so much better as a platform. Win Win.
I was thinking of re-purposing the Micro Server as a NAS with FreeNAS and retiring an old Netgear NasDuo, but on that basis the NasDuo, while not perfect, will have to get new disks!
In one go I took out 7% of my base load!
You could look into the “Watts Up?” meters that were made in the US. They are no longer manufactured, but you might be able to get one on eBay or similar sites. I think they can handle 240 V if you’re outside North America (but double check!). The .Net, Pro, and Pro ES versions all have USB ports thru which you can communicate in real time with a little bit of Python or C code to pull kW and kWh values. The .Net version has a relay that can be used for power switching. I’ve had great fun bundling a WattsUp meter with a RaspberryPi ZeroW to serve as a “smart energy monitor” for plug-in devices ranging from my refrigerator to sump pumps to plug-in AC units. I’m happy to share more about the Python code I’ve used if anyone is interested.
I have a Zhurui-Tec PR10 that works well as a power recorder.
I also use a Tp-Link Kasa HS110 wireless-controlled plug and that can log energy usage. It’s easy to adapt one of the shell or python scripts that are on the web to do exactly what you want. Mine logs readings into my emoncms system. I expect I’ll buy more of them if I need anything else.
I use the HS110 plugs as well. I use Node-RED to regularly interrogate the plugs and post the data to my emonbase using MQTT. Node-Red can also be used to control the plugs if you need to.