If connection is 18 days old, then outside the easier 7 day window.
In theory a 28 day window.
There is also a 28 day guarantee Virgin Media offer
"Money back guarantee: Available to new customers and existing customers taking a new service. Call us within 28 days of activation for a refund of your first month’s rental charges and any installation charge. You will only be charged for transactions (calls, purchases, service upgrades) occurring after activation. Statutory rights are not affected."