Thanks to your reponses I bought replacement fixings from Screwfix -
£4.40+ for five as opposed to Focus, which is much closer, who charged
£9+ for two. A dry fit proved that the bolt was too long - I'm unable
to drill deeper - but hacksawing 1/2" off sorted it and this time the
things worked perfectly. Metal plate bolted down solidly and fence
repair complete.
Everyone's advice much appreciated, pints all 'round ;-