Could be a new tank is required.
Parts diagrams here:
http://www.shouldersofshoreham.co.uk/index.php/9527151-90
http://www.manualslib.com/manual/357880/Flymo-Maxi-Trim-430-Parts.html?page=3#manual
http://www.shop.flymo.co.uk/petrol-grass-trimmers/maxi-trim-430-952715279-02/catalogue.pl?path=600082,159049:634291&model_ref=557707&illustration_ref=1082494&page=2
XXX, June 2016