Im fed up of my isis bb's only lasting a month or two, are there any long lasting isis bb's or will i have to upgrade to the external bearing type cranks?

I know ISIS has the problem of shorter bearing life, but that's pretty bad. I have a strong feeling doing something wrong. I'd check your setup, as I suspect you aren't installing them properly, so they have a little play and thus they wear out really quickly, but this is merely speculation as I've never seen the bike.

I suspect this because my really cheap ISIS BB (entry level Shimano I think) on my hardail is just over a year old and has silkey-smooth bearings, easily another year or two of life in it.

Shimano have not used the ISIS system – they used Octolink.
SKF BBs from Middleburn are about the best but I gave up after trying 3 different makes, Raceface being the worst & went 'back' to square taper.

it was fitted by my lbs and is a cheap transmission bb or something so your saying a cheap shimano one will last years?

my SKF one has lasted pretty well… I went for the all bells and whistles BFR-600 (stainless steel and a heavier duty spindle…)

It's only cheap (I've never actually bothered taking out the frame so not 100% sure on what make as it came with the bike new, but it was a Hardrock with Vs so it can't be an expensive one), but even then a year is the minimum I would expect from any BB (preferably more).

Even if your's is cheap it should still last more than a couple of months. Go speak to the LBS and tell them it's died, they should be able to sort something out.

My SKF has lasted two and a bit years now. The bearings are getting rough, but there's still no play

I had a RaceFace ISIS BB fitted to a Hummer nearly seven years ago. It was a Signature model. The bearings died after eighteen months, so the LBS fitted new ones, cost about £8, IIRC, then the BB was fitted to my Inbred 567 three years ago. It's still working just fine, and I see it lasting a good while yet. Even if the bearings crap out, replacing them's hardly gonna break the bank. I believe it's all in the installation.