They are scale insects. There are biological controls available, though I'm afraid I don't have any experience or details of those. Otherwise why not try dabbing them with methylated spirits, as this works well on citrus trees which are very badly affected by this pest.

Try washing them off using horticultural soap, or if you're desperate, ordinary soap, if there's too many for the methylated spirit treatment individually. After you've given the trunk a good wash with the soap and water, then rub over with methylated spirits on a cloth in the affected areas. Provado apparently makes a treatment for scale insect, never tried it, don' t know how effective it is, nor whether it has a significantly deleterious effect on the environment.