My broadband speed is much lower than expected

[#1]

I received an email from PlusNet today saying as it was 14 days since I signed up to their service, they are sending me a note of my expected broadband speed.  I have the 80 / 20 service, and the estimate my speed should be around 77mbs. 
I understood that it took some time for the line speed to settle down, but when testing the speed using the website speedtest.net, the average download speed is around 30-35mps, and a couple of times it has been much lower than this.
The BT engineer who installed my line told me I was about 30 yards from the cable box and there was lots of spare capacity, so I should be getting speeds near the maximum of 80mbs.
While the 30mbs or so I am getting is much faster than my previous service, I wanted to ask if this level of speed is the norm for the 80mpb service in my circumstances, or is there anything else I need to do to improve this.
Thanks,
Tom

[#2]

I have a suspicion what's happened but let me confirm.
[EDIT]
Turns out my suspicions were wrong. Are you running the tests over a wired or wireless connection?

[#3]

It's a wireless connection. I bought a new Billion dual band router for the new service, which I have been using since starting your service on the 2nd October. I have had speeds of around 75mbs for the first couple of days after the installation with this router, but over the last few days is generally much lower than this.

[#4]

Is there any chance you can try a wired connection?

[#5]

I connected my laptop today using the cable that came with the router and I was getting speeds around 70 mbps. I then tried my router again and found that when I used the 2.4 g band, the signal strength of my network adapter was showing at 130mbps and I was getting about 30mbps in the speed checker, and with the 5.0 g band, the signal was around 520 mbps, with a speed checker of between 60-70 mbps.
I had thought that with the 2.4g band showing a connection strength of 130 mbps, there was enough capacity to get speeds of  up to 70 odd mbps, but it looks like this is only achievable with he 5g band.