From memory the Topanga was a MTB? Any idea roughly how old it is, or photos? My recollection is that the Topanga was towards the bottom of the DB range, but a reasonable quality bike.
Diamond Back used to sell some very nice stuff, but all they seem to sell in the UK now are relatively low-end bikes. I think they may be linked to Raleigh somehow? Having said that,
the US site lists some high-end bikes
For £50 if it's in good useable condition and it's the right size you can't really go wrong.