I have already set up Column B with a validation list to select the 'Service'
and Column C a second validation list with a set of 'SubServices' based on
Column B.

The formuales are as follows:-
Column B
=IF(C5="",ServicesList,INDEX(ServicesColumn,MATCH( D5,MaterialColumn,0)))

Column C
=OFFSET(ServicesStart,MATCH(B5,ServicesColumn,0)-1,1,COUNTIF(ServicesColumn,B5),1)


Now I wish to extend this selection list to Column D which will provide a
selection based on Column C. Column D will be called 'ServiceSize', and shall
contain variosu pipe size information.

Looks like I need a MATCH criteria.

What formulae should I use for the Column D?

I have been playing aroud and have setup Column D to have a duplicate
'SubServices List' with Column E being the 'ServicesSize'.

Column D
=IF(D5="",SubServicesList,INDEX(SubServicesColumn, MATCH(D5,ServiceSizeColumn,0)))

Column E
=OFFSET(SubServicesStart,MATCH(D5,SubServicesColum n,0)-1,1,COUNTIF(SubServicesColumn,D5),1)

I need a method to join the 2 Column C and Column D together, or remove
Column D altogether. Any suggestions? Thanks.