I have had BT infinity 1 for about 6mths now,the first few months i had a constant. 36 Mbps now the speed on my laptop, connecting by wi-fi, has halved, from 36Mbps to 18 Mbps, i am testing the speed via the openreach speed tester, have done a virus check, free up disc space ect, any ideas, or a U.K. based customer service number, the Indian customer sevice is a nightmare.

You could try posting on the BT community forums, the mods can be very helpful, sorted things out for me in the passed.
My speed download speed was up and down a while back, from 38Mbps to 2Mbps and they got it sorted.
Bit of a problem at the moment I'm 32 Mbps 6 Mbps less than I was when I first had infinity but I'm putting that down to traffic and cross talk..

First off. Plug your laptop into the router via a cable and turn off the WiFi. Do a speedtest and compare the result.

If it is also low then chances are it is the broadband connection. If it is back to 36Mb then the problem is your WiFi.

Also what type of Infinity install do you have? Do you have a master socket with separate sockets on it for phone and the router/modem or a self install with plug in filter similar to ADSL? And do you have any extension sockets?

If you have a plug in filter and extensions then you need to eliminate your extensions as the cause. If you have an NTE5 type master socket then remove the lower section of the front plate and plug the filter into the test socket that is now revealed. If this is still slow then it could be a line fault. If it is back up to speed then it could be your extension wiring or something plugged into an extension socket.

It could very well be WiFi channel interference. I had this problem with my old Plusnet Router, and it sorted it (never have this problem the Plusnet Hub One, which a reskinned BT Home Hub 5a). As chrisjr said try plugging it in direct. If your speeds go up, then this could be the problem. It best to try Channels 1, 6, and 11 as they are less likely to get interference from other channels overlapping.
This explains channel interference (there is little diagram explaining how WiFi channels work).
Also pop over to the chaps at Thinkbroad, their forums are really good for internet based problems.

wouldnt count on bt helping you, i was getting 74 and now 55 dropped overnight according to bt thats acceptable because more people are connecting to the fiber cabnit that despite my guarteed line access speed is 64, had a openreach engineer out who said my line was fine really annoying

thanks to all that replied, will try the above suggestions.