Hi Edge,
The vermiculite concrete is an insulating layer to your oven, the concrete layer above it is thermal mass for the ovens cooking floor. Refractory cement is available from any of our cement manufactures in the UK, it is generally called high aluminium cement, ask for it or look on the cement manufactures websites. To build a brick oven refractory mortar is not necessary just use a high lime mortar mix.
You will find firebricks from the Travis Perkins web site, rather expensive, however, for a small amount like you will require they will be delivered so not to bad.
Alf