I once went into a radio shack when I was working on a low-pass filter circuit for a robot project. I asked if they had any trimmable (adjustable value) capacitors. The guy takes me over to the rack and selects a bag of caps with long leads:
"You can probably trim the leads on these."

I looked around for the "Candid Camera"
I couldn't believe it.

RS should have an 8-pin chip that is a "LED Blink driver" You wire up the resistor and the LED, and select the speed of the blink with an external capacitor value. Ask 'em for a trimmable capacitor. grin

Chip

Preaching the Hot Rod Gospel according to the 4-stroke apostles:

Suck, Squish, Fire and Fumes