See this link to order a duplicate CIGA guarantee for your Cavity Wall Insulation ...
If you are buying property, and the cavity wall insulation guarantee is missing, you should ask the seller to order a duplicate copy. Only the seller can order the duplicate.
Otherwise you will be responsible for the cost of the same and it could cause issues on resale or where you need to make a claim on the guarantee.
9 times out of 10 the Guarantee is with CIGA. A local search will reveal the same.
If the guarantee is not with CIGA there are a few other providers.