You asked about angling the bridge. Start off with the bridge straight. If the banjo's intonation is good on all strings, leave it that way. If the intonation is off, try angling the bridge slightly. If it's still off, consider changing to a compensated bridge. I don't know what's available in the UK, but here in the US Grover makes a compensated bridge that's very inexpensive. Other folks make compensated bridges that are quite pricey ($30 US - £22 UK). Too expensive a modification for a cheap instrument.

Bruce