Q:

Is mum/mam/mom etc ever capitalised?

When referring to a general mother, I'd always just use lowercase.

Our mums went shopping together. Your mum bought a new suitcase.

However, when addressing my specific mum, I couldn't decide whether or not to capitalize the M.

You'll have to adopt that dog Mam; we want to see it.
  When are you coming to England mam?

A:

Mother and its alternatives are capitalized when they are used as a form of direct address or a title (effectively in lieu of the person's name).

Mom, can I take the car?
'Night, Mother.
Cuppa tea, Mam?

but

I asked my mother if I could take the car. She said "no".
Your mum is a looker!

Reference to one's mother without a determiner (such as my) more often is treated like a title and gets capitalized

I asked Mother if I could take the car.

While some might use the lower case for this last example, this ngram shows such usage as much less common (thanks to FumbleFingers for this point).

A:

From the book 'Painless Grammar' by Rebecca, PhD
Capitalize 'mom' and 'dad' when you are calling your parent's name but not when you refer to 'my mom' and 'my dad'.
CAPS

Hi Mom! Welcome home, Dad

NO CAPS

My father and my mother are busy. Could your mom or dad drive us to the movie? 

