OFF: Slang question

[Vince LeGrand]

I know this is off-topic, but in the past this list has had threads on
various slang expressions.  A co-worker received a note from a someone in
the UK which included the following sentences:

>A couple of documents I mentioned during our telephone conversation.
>Apologies if you already have them and I'm teaching me grandmother to suck
>eggs.
>
My co-worker (and I) aren't familiar with the phrase "teaching me
grandmother to suck eggs."  Is this a UK expression?

Any help would be appreciated.