FLAC to MP3

Hi everyone. I’ve got a bit of a techy question. I bought a Brennan B2 and recorded about 1200 Cds, assuming they would be recorded in MP3 format. Unfortunately, they were recorded in FLAC and so I’ve run out of space. I’ve been able to back up the files onto hard drives. So, I need a programme for my pc to convert the FLAC files to MP3, which I can then reload onto my Brennan and have space left to record the rest of my collection. Anyone have experience of this, or similar, and can recommend a good free conversion programme for my pc. Thanks