Hi MrsSpratt, an old bath makes a good wildlife pond, as long as it has plenty of shallows (tricky with such steep edges). You'll need to seal the plug hole and overflow pipe using a pond sealant (available from aquatic shops). Then I would take some large stones or slabs and use these to create shallows (5cm deep). This is where frogs will spawn in spring.
Choose a sunny site for your pond and inlcude lots of native pond plants - such as marsh marigold, frog bit, hornwort and brooklime.
If you do sink the tub - which is fairly easy to do - make sure wildlife can enter and exit the pond easily. Consider sowing grass seed around the edges and allow it to grow long, which young frogs and toads will use to shelter in. Avoid filling the bath with tap water, but let it fill up naturally instead. This will ensure a healthier pond in the long run.
Hope this helps
Gardeners' World team