Hi Simon, I recently managed to have my own sustainable power supply cause I had the same problem with the 7Ah power tanks. They were a bit expensive and didn't provide sufficient power for the whole night. I'll tell you what I have bought. The setup is not so handsom "yet" but I tried it and it works perfectly.
1. 60 Ah semi-traction 12V battery. Can be in a range of €80. Your can set your own preference.
(Varta 60Ah accu Professional MF LFD60, 930060056, 12V)
2. Battery charger, cheap DUNLOP, slow charging but does a great job so far. between €15 and €20.
https://www.groupon.co.uk/deals/gg-groupon-goods-global-gmb-h-2340-158
3. Inverter DC/AC 500W. This is an amazing solution, will cost in a range of €50.
https://www.amazon.nl/dp/B07D4B1JXB/ref=pe_19967891_406998361_TE_SCE_dp_1
Bamo