@Blue Hills I'm Glad you made a choice & I hope it serves you well, that said, the same way you are hesitant of button cells is the same way I am hesitant of cheap D-locks, I wish it was pessimism, but alas, it was from my own experience that made the judgement call.
My Key had snapped in the cylinder of a cheap D-Lock I bought from amazon, all the cheap low-end d locks are usually the same Chinese import but with a different logo slapped on it. - The key snapped because the pins in the cylinder were seizing up (From weather?) Hardly put any force in the damn thing, the key was made of cheese, once more it hardly saw rain use. Just cold conditions and light use for about 6 or 8 months.
Credit to my luck (and rather my stupidity) i didn't lock it through the frame on my mtb, just the back wheel when the key snapped, so I was able to carry the rest of my bike home and then arrange transport to come back much later to "get" the rear wheel.
I tried digging the key out of the cylinder so I could use my spare key, but it wouldn't budge. In the end I couldn't be bothered, so I resorted to bolt cropping it the U lock. Medium sized bolt croppers & it went like cheese. - A little bit of resistance at first but then it just Glides through. Really surprised.
I reckon it was made out of mild steel. Nothing hardened at the price I paid, wasn't too upset about the whole experience but I realised if the key had snapped while the frame was locked up too, it'd of been a much worse experience.
Anyway, I presume its the £5.99 mountain warehouse lock that vickster proposed? I recognised it on Amazon a ways back, i was looking at it too.
Take a look at the reviews here, they are more indepth than on the mountain warehouse website.
https://www.amazon.co.uk/Mountain-W...ll_reviews&filterByStar=one_star&pageNumber=1