Hi, someone else, who's rootschat ID is daddikins, asked the same question in March 2005. I replied to them, but in case you get notofication of replies to your posts, I'll copy the reply to this post too -
Just in case you haven't found any yet, there's a way you can find out, at no expense (so that's good

).
In
www.ancestry.co.uk click on "search", then "England", scroll down to England Birth, Marriage & Death Records & click on "England and Wales, Birth Index: 1837-1983" NOT the FreeBMD one.
Type Brewster in the surname search window & I'd put 1927 plus or minus 5 years in the year windows (any more & it gets a bit unmanageable).
You should be presented with a list of at least 44 images to look at (4 for each year, or more if they're split between one page & the next). If there are fewer there may be some missing & then it takes a bit longer. Check out each image for the Brewsters. The lists after 1911 show the mother's maiden name, so any with the mother's maiden name as Grice are likely to be siblings of Mabel.
Once you've looked at 1927 +-5 try a few more years.
Good luck!
Janine
PS
I couldn't stop myself! I've checked 1922 to 1928 & I found a Robert Brewster in the 3rd quarter of 1928 in the Romford district. The ref is volume 4a page 866. Sadley it looks like he might have died aged 67 in the Redbridge district of Essex in 1995. If so, his birthday was 19 Aug 1928.
There were three other Brewsters born in Romford in that quarter, George J, who's mother's maiden name was Price & a set of twins, David H & Roy H who's mother's maiden name was Karwood. Maybe they were your mother's cousins.