Just a bit of an update - I used the ones that koolpc mentioned above. They did the job. Not sure how many uses I'll get from them though. Main problem is getting the plunger out after each use. Pretty sure I'll damage them (or me) at some point. You need a pretty strong stick or bar to push the plunger back out. I guess that's why they have the larger nozzles. I thought maybe the nozzle end might unscrew but not the case. Also they were slightly too long for my No Nonsense Heavy Duty Sealant Gun and had to be trimmed slightly. Bit pricey but seems to be a captive market.
Get one of your broken mastic guns, or buy a crappy one. cut off the piston bit, and use it back to front