Many devices today, EG. Blu-ray box and TV sets etc. have an Ethernet socket for a cable to BB router but in some cases a cable connection is not possible or convenient and the use of the routers Wi-Fi would be convenient to use. Snag is from my looking around these adaptors to put into the Ethernet socket of the unit do not come cheap, maybe there is a lack of demand.
Any cheap units or cheap ways of doing this?

Try PowerLine Ethernet adaptors. These only really work if your router and the devices you are trying to bridge to are on the same ring main circuit. There's also some question over the noise radiated by the ring main when being used in this way, though the jury is out on that specific issue.

How about this?

http://www.amazon.co.uk/TP-Link-TL-W...dp/B002YETVVE/

twenty three quid. Cheaper than most HomePlugs

I've used lot's of the Edimax EW-7228APn ones, VERY versatile units and can be used in a multitude of ways.

I thoroughly recommend the TP link access points.

I would also say though that the TL-WR743ND has 4 ports on it, so may be more useful in a world where your tv, boob ray, satellite box etc. could do with ethernet.

http://www.amazon.co.uk/TP-Link-TL-W...productDetails
and it's only £21
tp site here:
http://www.tp-link.com/en/products/d...del=TL-WR743ND

all access points do the same basic jobs. they can all be set as access points, range extenders, wireless bridge, or client mode. the latter being what the OP wants.

i've used a lot of them. i've found the tp-link to be the most stable. i have two and haven't touched them for years. they just sit there going their job.

Well, another twist on internet sex I suppose.

These units look universal then to me in that they can receive what my router is putting out and feed a unit via Ethernet socket.
Interestingly they seem able to other tasks like being a router, this leads me on to think can a router be arranged to be what I want a receiver adaptor?
I must have 2 or 3 routers spare in my loft.

All the instructions I have seen talk about turning an old router into a WiFi Access point which needs a cable connection back to the primary router. In other words it either converts a non-WiFi equipped router to WiFi (in effect) or extends WiFi coverage. It doesn't use the WiFi to communicate between routers

Several people have reported using custom firmware on the router to allow it to be used as a wireless connection to the primary router.

See

http://www.dd-wrt.com/wiki/index.php/Main_Page

and

http://www.dd-wrt.com/wiki/index.php/Client_Bridged