I was thinking of using the ring buffer technique with about 10bytes storage, when the value of the buffer was down to about 5bytes I would fill it up again.

Sinmce the simplification of ISRs is alway important, make the size of the ring buffer size a power of 2>

instead of

  .... offset++
  if (offset == size)
  {
    offset = 0;
  }
this becomes
  .... offset++
  offset &= (size - 1)

have a look at the generated assembler in both cases.

Erik