What were your Great grandparents names and birth dates. It makes searching easier with a few more details.

The ggf in question was Joseph Millin - often misspelt as Millen or Mellen - born; 1850 (June), died 1898 (December).
The story ran that he was a boiler-man at Siemans in Greenwich, & died cleaning-out one of the large boilers. It sounds rather sensational, so I imagined it might have been reported in the local press, but I haven't been able to find anything.
His widow - either Jane Sarah or Sarah Jane, Jerome (also Jeroms, Jeromes, Jeram or Jerams) was born in 1854 (April) and died in 1929 (March). Her husbands death left her, & their children, destitute - hence the Plumstead Workhouse.
Spelling would not appear to be the forte of this branch of the family!

This is possibly his death, the certificate will give you more details on how and where he died.

I found it on this site freebmd.org.uk

Deaths Dec 1898
Millin Joseph 48 Greenwich 1d 625

You can purchase a copy through GRO UK (General Registry Office)

This could possibly be their marriage, it will give you more details.

Marriages Jun 1880
Jeroms Sarah Jane Woolwich 1d 1227
Millen Joseph Woolwich 1d 1227

I found this Probate Notice
Probate
Millin Joseph of 32 Sand Street Woolwich Kent died 12 Nov 1898 at Seamans Hospital Woolwich Kent. Administration London 2 Dec 1898 to Jane Millin widow 10 pounds.

I wonder if the Seimans and Seamans part of the story has been muddled over the years. Died in Seamans Hospital Greenwich a boilermaker or worked for Seimans Greenwich as boilermaker, stranger things have been known to happen.

Have you tried contacting any of the local historical societies around Greenwich, they may have more information.