There is another option that hasn't been mentioned yet...
www.taydaelectronics.com
I've got a few of these little trimmer-caps in values up to 60p. I got mine from the local shop, which doesn't have the larger values (70p, 120p) that Tayda has (which is where I get most of my bulk-order items). I'll be ordering a few 70p and 120p from Tayda in the next round.
Use the 50p trip-cap dialled all the way up, or 60p trim-cap dialled back to 51p.
I paralleled a 60p trim cap with the stock 30p slew-rate cap in my Filigree Siberian Hamster build (*It's a RAT, Manuel).
I've also got some 51p MLCC kicking around, but may have gotten those at the local shop or at Tayda, can't remember which.
Newark has them in stock, but I've never ordered from Newark and don't know what that company is like to deal with.
Not really worth the shipping cost unless you bulk up on some other items, too.
If not the Trim-cap mentioned above, the Tayda 50p Ceramic Disc cap is closest to the 51p value desired. Since it's in the audio path, a Silver Micah, as already mentioned by Fig, would be a good way to go.
Other ways to go about it is running a couple of caps already on hand to get closer to the target value, either in parallel or series:
Series 220p + 60p = 51.9p
Series 820p + 56p = 52.4p
Calc'd here:
http://www.1728.org/resistrs.htm