NEW Fibre Connection..half the speed it should be :(

[#1]

We had our home 40mb Fibre connection put in onFriday
Engineer's gear said we should get as near the 40mb as possible as we are just round the corner from the cabinet.. I'm afraid not!! 21mb has been the best speed test yet, i'm not going to lose sleep over it but if there's something we can do to get the best out of it i'm all ears..
The router is right beside the fibre box in the house so there's no issues there and i'm using the standard issue Plusnet Router(it's soooo basic)
Any ideas? Should I be waiting the 10days before reporting to PN?

[#2]

Is that with an ethernet cable to the router?  WiFi can easily struggle to keep up with FTTC speeds whether it's '54Mbps g' or '150mbps n' you'd be surprised how slow it can be.
If you use the BT Speedtester (speedtester.bt.com) it should tell you your line profile, that's about the only way of confirming your sync speed with the locked BT OpenReach modem (sync is about 3% more than the BT line profile).