A magnetic compass is really useful for this purpose (and hiking) and you can find cheap ones at Decathlon or Amazon.uk (less that 10£)
The paper arrows are not necessary, I made them with a inkjet printer and simple A4 paper, but in your case, do not make them. Just walk around and check consistency of the indicated ‘north’.
If not consistant, just take note of the min/max difference (in my case on the BBQ with pictures above I found a 360° difference, but on the car roof, I found +/-40° difference (for a total of 80° ... and that’s a lot !)
