Can I refuse a secondment?

Yes, unless your contract allows your employer to require one and the request is reasonable. Ask to see the clause being relied on and reply in writing.

Does a secondment break my continuous service?

No. Your employment continues with the same employer, so continuous service runs on for redundancy pay and unfair dismissal purposes.

Who handles a grievance during a secondment?

Normally your own employer, because it holds the contract. The secondment agreement should say how complaints involving the host are dealt with.

What if my old job is gone when the secondment ends?

Ask in writing what post you are returning to. If the role has genuinely disappeared, redundancy consultation should follow rather than a quiet move to a lower role.

What are my rights on a secondment at work?

A secondment is a temporary move while your employment continues with your existing employer. Your contract, continuous service and statutory rights carry on throughout.