Complex sentence: There is no formal membership, but the EDL's former leader, Tommy Robinson, estimated a membership of around 100,000, but the think tank Demos estimates that there are between 25,000 and 35,000 active members.
Split into simpler sentences: There is no formal membership, but the EDL's former leader, Tommy Robinson, estimated a membership of around 100,000. The think tank Demos however, estimates that there are between 25,000 and 35,000 active members.