well I don't really like 15 inch speakers that much, too much low end. 12 inch is the perfect blend of bass and high end for my taste.
Also, at 50 watts it's gonna be a pretty clean sounding speaker, it will probably sound pretty good with your amp distorted, but you're not gonna get as much speaker distortion from that thing which is part of getting a good drive tone. You need pre-amp, power-amp, and speaker distortion. How many channels does your bogner have? And is one of them a drive channel?
If it's a one channel and the only way to get it to breakup is to crank it then that Celestion probably isn't a great match, a Celestion Greenback (25 watts) would probably work better as it's lower wattage and has a more mid heavey spongey sound that works better for distorted tones. But if you want clean tone then maybe the 15 inch celestion is a good choice, you won't know until you try.
And at 30 quid you're really not out that much cash, just the time and effort of installing it.
