a photo mite be a good idea .

Is there any sign of resin ouzing out? Sounds as though it might be canker - try googling canker plum images for some pics.

Hi, as plum trees are susceptible to silver leaf fungal disease, I would suggest you get some Arbrex seal and heal from your local garden centre, or elsewhere, and paint it on the split, do it in dry weather, it should not affect the fruiting, Derek.

Thank you all, I`ll take your advice and see if it can help the problem.
Thanks once again.