[name]
The Headmaster
[St. Cake's]
[etc.]
Dear Mr. (or Dr., etc. - double-check) [name],
I have seen your advertisement for a [ ] in [name of paper] and would like to apply for the position.
I enclose a copy of my CV. You will see that in terms of employment my experience has generally been in retail, but outside work the sciences and, in particular, electronics have always been a great personal interest. [Put in just one example, practical if possible - eg for the last 3 years I have been helping the local scout group with the practical work they need for their merit badges in electronics - etc.]
Your position would be an ideal opportunity for me to transfer my interests into paid work, and I would hope that you would be able to consider me for the job. I look forward to hearing from you.
[and beef up the "interests" bit of the CV to put in everything that supports the letter, including maybe some names of referees who can vouch for the interests]