In reply to blackmountainbiker:
A subordinate clause is a clause that doesn't work by itself. e.g. "If they could afford to." should not really be a free standing sentence (you can use it as an answer to a question "Did the Romans eat well?" but it assumes a main clause that should go with it!).
A conditional clause is one kind of subordinate clause (like a temporal clause, concessive clause etc.): the mark scheme should accept both answers. It is a conditional clause (because of "if") and it is therefore also a subordinate clause.
If we are hitting higher level pedantry, I think "conditional sentence" is better than "conditional clauses", because in a conditional sentence the verbs of both clauses are affected. For example, the open condition: "If they could afford to, the Romans ate well"; closed condition "If they had had the money, the Romans would have eaten well." First indicative, second subjunctive.
All this is very Latin based. I'm sure an English teacher would explain it differently.