I think you are copying the wrong url. When I use the following, it worked.
url_base = 'https://www.og.decc.gov.uk/information/wells/pprs/Well_production_onshore_oil_fields/onshore_oil_fields_by_well/0.htm'
Attention: it is .../onshore_oil_fields_by_well/0.htm
,
instead of .../onshore_oil_fields_by_well/0.html