Another userdata question

I am really hoping someone could help me with this question please as I am totally stuck on it for now.

I am trying to implement a userdata via the C API, to give me this type of functionality

btn1 = graphics.newButton(x,y,width,ht, "pressMe")
btn1:setText("Different Text")
btn1:setFont("Arial")

That's OK I think I have got that figured out and largely working, but the next thing I want to do is call a user defined Lua action function on the button instance. For example

In Lua

function btn1:action(someArbitrayParamBackFromC)
     print("ouch someone just poked btn1")
end

How can I call that Lua button instance function from C when I have detected the key is pressed ?

I have done something vaguely similar in the past when I have created a custom timer on the Lua side, but the difference was I had a timer create function in Lua such as

myNewTimer = createTimer( period, luaFunctionCallback)

So I was passed the callback function into the C code which meant I could stash it away in the registry and then when the timer elapsed on the C side, I did this

// push function indexed by registryRef from the registry back onto the stack
// the lua callback function in this case
lua_rawgeti(local_L, LUA_REGISTRYINDEX, registryRef);
/* the first function argument is the timerID */
lua_pushinteger(local_L, myTimerID);
/* call the function with 1 argument, return 0 result */
pcallResult = lua_pcall(local_L,1,0,0);    // pops 2 params from stack

I am guessing the solution is somewhat similar but cant figure it out.

Many thanks for any assistance

   Geoff