How do I get rid of the Net Shield pop up ad

My iPhone safari app is blocked by a pop up ad for Net Shield. I am take immediately to 'safari-extension.com' where this message appears: 'We have detected a malware advertisement (e.tre456_tech_iOS) on sites you have visited.' There is no way to refuse this - if I cancel it, it takes me to th eApp store, , if I install it's telling me I will owe £89.99 a year!

I can't access Safari and it's telling me I may have inadvertently visited a site that has put my phone at risk from cyber criminals: have I already done that? Are these the criminals?!