JCT - Client changing specifications - what can I claim?

The contract is a fixed cost jct the client is out sourcing different specifications from the contract i.e. Cheaper bathrooms cheaper floor finishes and external cladding we are still applying ohp to the new specifications but the changes are having a huge impact on the over all contract value. Can I claim the original ohp from the original specifications I understand the Client needs the right to change there mind on specifications

I assume you mean the original OHP as a lump sum. The reality is probably not but in pricing the change as you describe you would be entitled to adjust the OHP percentage if the nature of the work is changing significantly. You may also need to think about an increase in productive supervisory costs (formen etc who would be part of the OHP) depending on the nature of the change.