Problem removing outdoor wall light

I want to replace an outdoor wall light but I cannot remove it from the wall. It is a modern stainless steel lantern on an arm with a circular base attached to the wall. I assumed that undoing the two bolt heads on the base would release it from the backplate but it does not. There must be some other mechanism keeping it attached. What could that be?
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Could you link to a picture/give make/model?
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wrote:

It was purchased in B&Q about 8 years ago. I don't have any info about make/model but I have a photo of it here:
http://uploadingit.com/file/gskgmypadgzzzdwp/large_walllight.jpg
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Removing the screws should replease the cover, not the whole assembly. It could be that there is corrosion inside the cover which is stopping it being removed. It looks like a bit of BF&I might be necessary.
--
Woody

harrogate three at ntlworld dot com
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