Hi
First of all approach the Insurers of the main residence who should be able to help.
Try the Royal Bank Insurance companies,e.g.Direct Line, Churchill, NatWest, RBS who may offer cover subject to the details you provide and acceptance of terms and conditions. The second home will have requirements applied restricting cover and requiring reasonable security.
Having said this, I think most companies will offer cover in these circumstances. If they refuse perhaps it is down to other risk information supplied or lack of knowledge of the person your mother has spoken to. Ask to refer to a Team Leader if you have a problem. Perhaps think about going to a broker who may be able to obtain a better deal. Find a broker using the BIBA site. http://www.biba.org.uk/ConsumerHome.aspx .