Go to mother care. i trie a few and their was about 4 that fitted, it was then just up to me to choose. I even fitted a britax asis first class si, Was a bitch to get my son in and out but it did fit!
http://www.google.co.uk/imgres?hl=en&sa=X&tbo=d&biw=1163&bih=638&tbm=isch&tbnid=ryMQBcxrHvpstM:&imgrefurl=http://www.britax.co.uk/car-seats/first-class-plus-special&docid=AcshVYSnEGq1UM&imgurl=http://www.britax.co.uk/images/abb_first-class_top.jpg&w=617&h=267&ei=aZr2UKqcO-fY0QXO9IHoBw&zoom=1&iact=hc&vpx=4&vpy=300&dur=1864&hovh=149&hovw=342&tx=261&ty=80&sig=115454351309069015161&page=1&tbnh=112&tbnw=259&start=0&ndsp=20&ved=1t:429,r:6,s:0,i:100