I'm looking to get some B2 boots, but am having difficulty getting a good fit. I tried Scarpa Manta's but my heel kept lifting. I think the issue is that my ankles are quite narrow. Does anyone have any good advice for getting better fitting boots or is it just a case of trying a range of brands? Are some brands known to make narrower boots?
While some brands may be known for narrower boots, this also depends on the last they use for a particular model. La Sportiva boots are often considered to be narrow and low-volume compared to Scarpa, but there are also differences between the lasts they use for their Trango vs. their Nepal lines of boots. (The Nepal last is wider than the Trango.)
If you aren't able to find a model that works, you may be able to improvise. A few weeks back my girlfriend's bootfitter added foam under the tongue of her ski boots to help reduce her heel lift. It occurs to me that this the same idea behind the extra tongue supplied with the Nepal Evo's, and so you might be able to do similar with whichever boot you end up buying.
One final idea you might try is using a women's boot (assuming you are male). La Sportiva's FAQ says "Our women’s models have a wider forefoot to heel ratio compared to our men’s shoes and are narrower, straighter and lower cut heel and have less volume overall".
 A roughly foot-shaped form used to design the boot.