I have a SRX running OSPF with the rest of the network; on one of the interfaces it links to another part of the network with a large address space coming across the OSPF routing table.
>>Are they OSPF routes or routes from other protocols?
A few options:
1. OSPF Import/Export policy. Because your description of issue is not clear, not sure which one will fit your situation
2. This might also be acheived by OSPF Stub/NSSA area based on how you want to design