It's literally just hit me that I haven't yet received my logbook for purchasing my car around 4 month ago!
My tax is due for renewal at the end of this month. I have the green slip.

To tax the car at the post office what documents do I need to take?

Get onto DVLA first and chase up the V5, I'm assuming that you got the car with 6 months tax so you've plenty of time. IIRC you need the V5 and the MOT certificate to tax at the post office, or the tax reminder which you won't get until you have the V5 as it'll be sent to the previous owner's address.

You say 4 months ago.

I really hope for your sake that you bought the car before October the 1st last year.?

From October the 1st 2014, road tax wasn't transferable, so you may have been driving around without any.

You can tax it online using the 12 digit code from the green slip.

If you don't have the tax reminder in the post, then there's a good chance that DVLA are not aware of the vehicle transfer, so applying online with the 12 digit code should initiate this automatically.

Yes I bought the car on 27th September

I have printed a v62 form off to get the log book. I'm still in constant contact with the Dealer so shall get it one way or another!

In which case, I'm almost certain that if you apply for your tax online using the 12 digit number on your green slip, then you will be sent the V5C automatically.

As it's being done the wrong way around (so to speak) then usually DVLA will send you the V5C and a letter to the previous registered keeper notifying them that someone else has applied for the V5C.

I wouldn't bother involving the dealer, just deal direct with DVLA.