Seems to be quite a lot of sources for Cossum Hastings on the National Archive Site - for example:
Copy will of John Cossum of Hastings, gent HOP/43/1 4 May 1773
These documents are held at East Sussex Record Office
Contents:
Leaving real estate to his three daughters, Elizabeth, Winifred and Sarah Cossum and pecuniary bequests to his wife Anne, nephew Richard Cossum and daughter Ann Gilbert
Not sure this is the right Richard, of course...unless this John married twice...
Thank you for the info, It doesn't seem to fit but they may be related somehow.
GeneT