Unable to see my Ads - BT ISP

[dtprojects]

Two of my clients are complaining that they can't see their Google ads yet I can. I am using Sky Broadband in West Sussex and both my clients are using BT, one in Huddersfield and the other in Leicester.

The ads are UK targeted and we have never had this problem before. Traffic is down, as are sales and I am concerned that BT server IP addresses may be appearing to come from outside the UK. Anybody else experiencing this?