What’s the procedure here?
Local sub-post office didn’t want my parcel when I revealed that there was a lithium battery inside, but they couldn’t offer much advice on how to get it through the system.
I’ve looked at RM guidance- is it simply a matter of packaging securely so there’s no chance of bridging the terminals and putting a warning label on the outside? Can I just print a generic one off, or do RM have their own. I’ll have to take it to the main PO in town now, but don’t want to get turned away again..

Post office will take them , you just have to tell them what’s in the package and they put a sticker on the parcel .

I’ve just had the mother of all Lithium battery’s delivered to me by Parcel Force

Is the battery in something?
When I’ve sent lights with lithium batteries and they’ve asked what’s in it I’ve just said bike lights and it’s been waved through, never caused any problems so assume it’s safe.

is it simply a matter of packaging securely

AFAIK, no.
Lithium batteries with electronic device* = Royal Mail restricted items (ie, can be sent with RM under certain conditions)
Lithium batteries in isolation = Royal Mail prohibited (ie, use other courier). (source)

as a resident of n.ireland i can’t get them mail order. but if i order a battery with an item that takes batteries , its no problem. boils my piss so it does

“oh, yes, it’s a book I’m posting”