TL;DR Who (in the UK) are a reputable third party manufacturer/supplier where I might be able to get a 91Wh replacement battery for an M3800. 2-Power, Duracell and Green Cell only do the 61Wh variant. Amazon list a genuine Dell 91Wh one but it's 160 quid and I don't trust the reviews of the 50 quid ones. The last one I bought from ebay and it was rubbish. If anyone's interested I did a bit of research on the Dell website (using google site:dell.com) and found out that the 91Wh battery Model Name is 245RR and part number 7D1WJ, it's 7810mAh. T0TRM/H76MY/Y758W all relate to the 61Wh/5180mAh variant - most suppliers just use all these numbers interchangeably.