Update: I managed to take a sample of fuel today. I undid the sender unit as advised and was able to pull it out far enough to allow me to get a tube into the tank. I ended up using one of those really cheap siphon devices with the red squeezy bulb. It worked a treat.

I’m happy to report that the fuel was clean and clear without any obvious contaminants.

I’m puzzled by the presence of a thin, translucent tube lying on top of tank. It’s about 10cm in length. Each end is attached to a connector that goes into the tank. I have no idea what it’s for. Any suggestions gratefully received!

Again, thanks for the advice.