Toilet not flushing but not clogged

[spanner]

Ok.

 

[heliotrope]

Anyone think it may be a siphonic pan playing up?

 

[JusKel]

If it's Cast iron then it's almost certainly blocked/obstructed (like arteries of the heart leading to a heart attack or full bowl in your case) with just enough space remaining for a slow trickle or hose to empty tru.
As the vent section is no longer used its clogging itself up deteriorating.

 

[Welder]

Anyone think it may be a siphonic pan playing up?

Now you mention it Ive seen many replaced with normal syphons. some work aftervavfashion but many don't...

 

[justlead1]

As Heliotrope has asked siphonic pan is a must to check it fits in with the dates.

 

[Ric2013]

As Heliotrope has asked siphonic pan is a must to check it fits in with the dates.

Can the OP send us a side-on photo of the pan then please?