Damn - I know the feeling. I had my Scott Pro Racing blasted and Cerakote'd, applied new decals and then noticed the surface rust on the underside of the chain stays has actually turned in to a pin hole... which then opened up a bit more once I poked it.
Hoping I can get a repair done like the video above.
But yeah I'll echo £300 is a lot for a powder coat, especially with that BB finish. I think the last frame and fork I had done was £100 and I think that was in Cerakote with an additional clear. Granted it's just a regular frame but still..