So I'm currently setting up a new project and I've realized when creating adhoc and appstore builds. Fastlane doesn't seem to be using the existing certificate which created a couple months back. Instead, it seems to be generating a new certificate and using this to sign my profiles is there a way to specify/force Fastlane to use the old certificate rather than the new ones?
You can do it, but not simply, since this is not supported by fastlane match. You have two options:
First, you could just use the get_certificates (previously cert) and get_provisioning_profile (previously sigh) actions instead of match (now called sync_code_signing). This gives you everything except the Git repo syncing.
If you want the Git repo syncing (you probably do), then you can manually construct a repo identical to one match would create, but using your existing certs (see Manually Manage the fastlane match Repo). The basic trick is to create a new repo in precisely the correct structure by following the match source code, and then use Match::GitHelper.commit_changes to encrypt it. It's a pain to set up but from there you should be able to sync seamlessly.
Alternatively, you can use sync_code_signing nuke to destroy your existing certs and regenerate from scratch. This will get match working at the expense of other teams who may be relying on your old certs.