LED upgrade on existing 12V transformers

[DIYinBerks]

A friend of my wife asked my advice on this topic: I was not sure of the answer but said I knew a place to ask...
Problem is that her kitchen halogen downlights (50W) were installed buried in insulation and consequently overheat badly; some are now brown and several transformers have failed over the years. She was quoted £1500 for an upgrade to proper LED downlights with integrated transformers (gold plated I assume at that price!) which would be much lower wattage and solve the overeating problem. BUT, she also picked up in B&Q a 4.5W LED advertised as a plug-in replacement - I have seen these in Toolstation etc.
My question is this: the existing transformers are marked 20-65W but the LED is only 4.5W. It seems to work. So what does the marking on the transformer really mean? The LEDS are marketed as plug-in replacements for halogen bulbs so presumably it isn't expected that the transformers would need to be upgraded to LED-specific. Or do they?
As a supplementary, what would be a reasonable quote to replace 8 downlights with "proper" LED units. I assume that would be a part P job as they are in the kitchen?

 

[PrenticeBoyofDerry]

If you could post a link to the plug in LEDs, it would be helpful to determine what the product is and whether it is what it says on the tin.

The problem you have is your insulated ceiling void, as the transformers also require ventilation.
Replacing the downlights with reasonable quality fittings and lamps, providing the the existing cable is in good condition.
Would be about £300-£350.

 

[Iggifer]

http://www.lighting.philips.co.uk/pwc_li/gb_en/connect/tools_literature/MasterLED_April2012.pdf

there's a little bit of info on the technology behind the technology on page 20.

I removed an over insulated failed downlight yesterday, the wiring was nice and burnt and slimy. Was lovely

 

[DIYinBerks]

Interesting datasheet from Philips - looks like one of those products would meet our friend's needs exactly. Pity they don't explain how they do it...
Here's another similar (though I think they have MR16, need to check):
http://www.toolstation.com/shop/Sylvania+LED+5.5w+(50w)+Dimmable+Lamp+GU10/p44304
...so it seems like the answer is that not all LED's will work with transformers designed for Hologen - but some will. Might be more expensive than a LED designed o work off a LED driver.

 

[DIYinBerks]

Just a quick question for PrenticeBoyofDerry:
Was that price for labour only or for the complete job?