If you've got £300+ to spend, try and find an old AKAI Deep Impact, or for £60 get a Digitech Bass Synth Wah. I have the latter and it can put out some awesome sounds. It has a mode where it has 2 effects on at once, a sub-octave plus an envelope filter, put that before my £20 Danelectro French Toast fuzz pedal and you have a wicked effect for DnB.
Electro Harmonix Bass Multi Synth is also meant to be a good one, but not heard myself.
Zoom B2.1u has synths and with an expression pedal which could be interesting, but not sure how well they track your notes or how good they sound. I have a Line6 amp with built in synth and an external FBV express expression pedal, but the tracking is horrible.