Office 365 requires 2 SRV records set in the DNS for full Lync capability. It is not obvious how to set the correct SRV settings in Fasthosts advanced DNS configuration page as there is no Protocol field to fill in.
It turns out you need to append the protocol to the host name as follows.
|Host Name||Points To||Port||Weight||Priority|
For full details see https://help.fasthosts.co.uk/app/answers/detail/a_id/1727/~/introducing-dns-srv-records