[AccessD] Cannot delete Missing references

Yesterday I was out on-site with a client and together we made some changes to the mdb. I burned a copy of the modified version to bring home to continue working on and now the references are causing me some problems.

Client had Frontpage refs which are newer than mine - so naturally when I load I get 'Missing' for the 2 front page files. I tried to just untick those refs and then add my versions but I simply cannot untick those missing references and click OK - I get error message 'Cannot perform requested operation'. The other thing I tried was to replace the refs by clicking browse and override the missing refs with mine - same error.

BTW - I thought maybe an open form or something may be blocking me from changing the refs - but I am just shift+opening the mdb - no open forms or anything else.

What am I missing? 

TIA

Kath