Newbie Post Migration Speed Query (No Fault)

[#1]

So my migration to PlusNet on Friday went exceedingly well - everything happened as it was foretold, and very happy with the workmanship of the Openreach engineers - they even fitted the cabling outside the house.
OR Engineer showed me line speed on his little tester gizmo which read 76mbps - my understanding is that is pretty much a perfect score.
I then connected my laptop to the PlusNet router, had my 15 min cuppa and hit a Speedtest - some 69+ mbps (a couple of times) - very happy bunny.
Especially as the estimate was for 55 to 56 mbps.
All good
Set up the wifi
Set up my homeplugs and off we went, zippy zippy.
Of course, I've been doing some more speedtests - via the wifi & homeplug wired network but now the repeated results bump along at just under 40mbps.
Still 10x faster that I was getting - and I understood the warning from PlusNet that it could take 10 days to settle down, even though the OR engineer this was more like three days in reality.
So a bit of browsing got me wondering if I'd been mistakenly put on a 40mbps profile by PlusNet - it apparently has happened.
Anyway, this afternoon, I got around to connecting my laptop directly to the Technicolor router, speed test and bang 69mbps
Immediately onto wifi and back down to 39 ish.
Signal strength was "excellent"
So my questions are - I bet you never thought I'd get around to them.
Is the significant speed drop off due to some inherent wifi overhead?
If so, can someone explain, in non tech language, why.
Homeplugs seem to be slower by 1 or 2 mbps but broadly comparable.
Is there something I can tweak on the Technicolor router to improve the wifi and/or homeplug speed?
I appreciate a "free" router, it's not the best perhaps but wifi signal seems to be strong around the house. Would buying a "better" one improve things?
Again I repeat, I'm happy with my dramatic speed but it seems a shame not to benefit if my line can do better.
A link to my Speedtest history:
http://www.speedtest.net/results.php?sh=93f0cec983e27747be02232e9d2f6bac&ria=0
The first three are the final ones from my old Enta connection
The first spike is just after migration had completed, direct cable to router
Then a myriad of tests over the past few days
The last spike was this afternoon, again direct connection to the router.
Thanks

[#2]

Quote

Set up the wifi Set up my homeplugs and off we went, zippy zippy.

And there is the reasons why you are getting less throughput  homeplugs & bb IMO are not a good mix, i would avoid , nothing beats a quality cat6 ethernet cable
no tweaks will improve throughput ,

[#3]

You can try changing your wireless channel to see if that helps?

[#4]

Thanks for the responses: (apologies for long rambling post)
I was aware that WiFi was slower than Wired but Googling suggests it can be 30%, even 50% slower due to multiple reasons. I wasn't aware that there might be such degradation. But if true, it seems I'm operating within expected parameters.
I did try channel hopping - I understand that 1,6,& 11 are usually recommended, I'm on 6, but although I didn't try every one, I didn't see an improvement (though one was markedly worse!)
There are a couple of other things to try - such as relocating my dect phone which I understand operates on the same 2.4Ghz frequency and maybe shifting the router a bit. I can't do much about interference from fridges etc!
I had a quick look with Inssider and it labelled weak connection, low signal strength (it's about -55 to -65dB other networks around seem to be registering sub -90db), strong overlapping networks and strong co-channel networks - it's amazing the speed I get with all that against me  !
So without spending money, upgrading equipment such as routers, antennas and WiFi cards, I think I'm unlikely to improve things much as the speed variance is pretty much as to be expected.
I'm not sure why I'm feeling a bit "miffed" at this.
I was happy enough for years with my old ISP which delivered less than 4Mbps, and now I'm getting nearly 40Mbps for slightly less than I was paying!
I guess it's like being used to walking everywhere at 3mph and then getting into a car but being frustrated at only being able to drive at 40mph despite the speed limit being 70mph because of congestion on the M25 !!!
It wouldn't actually make much difference if I upgraded from a Ford to a Ferrari 
Edit:
I just had a play around, disconnecting the dect phone & homeplugs - without discernible change - but I also tidied up the power, plugging everything into a four socket extension lead & slightly moving the router in the process.
WiFi speeds have increased by about 30% to just brushing 50Mbps
I have no idea if it was re-booting the router & modem, or what but I'm happy with the increase - especially as it's within touching distance of the original line speed estimate.
Perception is a funny thing
Miffed at getting 10x speed, happy now because I'm nearly at what was estimated, not bothered about squeezing the maximum out of the connection because of extra expense - like spending to get your engine chip remapped for a few extra horsepower.