Youâre right, my apologies - just did that with curl and I see the relevant tags.
Currently you have
<meta name='twitter:card' content='summarylargeimage'>
<meta name='twitter:site' content='Ladbrokes'>
<meta name='twitter:title' content='Ladbrokes Casino Deposit Match'>
<meta name='twitter:description' content='Join now for a deposit match up to ÂŁ500'>
<meta name='twitter:creator' content='Ladbrokes'>
<meta name='twitter:image' content='http://socialicity.co.uk//Image/GetThumbnail/3396ba3f-ff42-45cd-8dd2-57237f995ea3'>
These look correct in the sense that you have a set of tags requiredâŚ
One thing you need to change is summarylargeimage to summary_large_image to match the documentation on this card type. Iâd also recommend updating your single quotes for double quotes - not required at all, but that would then be consistent with the rest of the code in the page 
Try that change and see if you are able to get the preview rendered?