Errors Faking Sharepoint with Typemock due to assembly versions

I was doing some work today where I needed to fake out a SPWeb object. No problem you think, I am using Typemock Isolator so I just use the line

var fakeWeb = Isolate.Fake.Instance<SPWeb>();

But I got the error

Microsoft.SharePoint.SPWeb.SPWebConstructor(SPSite site, String url, Boolean bExactWebUrl, SPRequest request)
Microsoft.SharePoint.SPWeb..ctor(SPSite, site, String url)
eo.CreateFakeInstance[T](Members behavior, Constructor constructorFlag, Constructor baseConstructorFlag, Type baseType, Object[] ctorArgs)
eo.Instance[T](Members behavior)
(Points to the SPWeb web line as source of error)
TypeMock.MockManager.a(String A_0, String A_1, Object A_2, Object A_3, Boolean A_4, Object[] A_5)
TypeMock.InternalMockManager.getReturn(Object that, String typeName, String methodNAme, Object methodParameters, Boolean isInjected)
(Points to line 0 of my test class)

This seemed strange I was doing nothing clever, and something I have done many times before. Turns out the issue was the version of the Typemock assemblies I was referencing. I have referenced the 5.4 versions, once I repointed to the new 6.0 (or I suspect older 5.3 ones) all was OK.