yep the ../../ thing applies to entries in desc_items
I found my problem finally....
the vanilla version I had been copying things off had
greek_town_gateway
{
full_health 40
localised_name gateway
level
{
min_health 0
battle_stats flammability_none impact_damage_low
item greek_town_gateway
physical_info info_walls/info_greek_town_gateway.cas
in greek walls

it seems that info_greek_town_gateway.cas
either didn't work properly initially anyway - not sure how much of greek walls are used in vanilla - or certainly doesn't work with models I'm trying to use.

switch to
info_walls/info_roman_town_gateway.cas
seems to fix the problem

(must remember to write things down or post them as they occur in future - would save lot of time!!!)