I just checked, 19 Management does manage songwriters and producers - says so in their blurb. I also notice they have Cathy Dennis on their books (although that may be from her performing days).
http://www.19.co.uk/site3sm.html
However, I would anticipate that it would be easier to interest a publisher than a management company because when you go into management there's more add on expenses, while there are many reputable "1p. publishers" who are happy to make lots of pennies. The advice given by Dolores and others is the best advice
- go for the publishers, unless you are thinking of performing too.
Songwriting, recording and creative & links
http://www.angelfire.com/music2/richhoncho/index.htm
A selection of RMMS at
http://www.angelfire.com/music2/richhoncho/RMMS.htm