http://www.motorcycleparking.com/
http://www.parkingforbikes.com/
Weekday afternoon? Gonna be tough for bike bay spaces. The best thing is to make a small map and mark a couple. That way you can keep riding round till you find one. Try Golden Square as there are 4 bays there and it's quite close to Trocadero. Alternatively, NCP charge something like £3 (?) for a whole day. Might be easier.
Remember, if the bay has a meter and someone has their car in it, you can park in the bay too. It's hard for them to prove it wasn't you that paid.
Enjoy the bowling!