I started with a regular drill bit and got one hole done successfully, give or take a bit of unpleasant screeching and smoke. The second hole went nowhere with the drill bit quickly dulling to the point where nothing happened. Given that the tank was made from thinnish sheet, maybe 3mm thick, I was surprised.
A bit of googling enlightened me. Apparantly stainless steel work-hardens, which in turn overheats standard drill bits and blunts them. The solution, apparantly is Cobalt drill bits, a slow drill speed and loads of cutting fluid.
A trip down to the trusty Screwfix to buy some bits and I tried again. An unbelievable difference - like a hot knife through butter. It's amazing what you learn along the way.......
The first pic shows the holes drilled in the tank top.
I neglected to mention what I saw when I peered through the newly cut hole, but I save this for another post.