DAB radio broadcasts are transmitted in 'multiplexes' , which are 'bundles' of radio stations, in the frequency ranges mentioned by Chuck above. Your DAB radio will automatically select the most powerful multiplex when receiving Heart FM. (Because multiplexes often cover quite a large area, the most local version of a particular station might not always be available on DAB). A DAB radio seeking commercial radio stations in your area will either take a signal from the 2000 watt transmitter at Whitehawk Hill or from the 1000 watt transmitter in Eastbourne, depending upon whichever provides the strongest signal at your location
However, as been suggested above, many DAB radios can also receive VHF (FM) frequencies. (So, even though it's a DAB radio, you might not necessarily be listening to DAB broadcasts). Because I'm too lazy to get a map out, I'll simply give you the relevant VHF (FM) frequencies to try.
Heart FM Sussex transmits on the following frequencies:
102.4 High power (8200 watts) from Heathfield (for the Eastbourne area)
102.7 Medium power (3600 watts) from Reigate (for the Reigate & Crawley areas)
103.5 Low power (900watts) from Whitehawk Hill (for the Brighton area)
102.0 Very low power (200 watts) from Hastings (for the local area)
96.9 Very low power (100 watts) from Newhaven (for the local area)
97.5 Very lower power (30 watts) from Horsham (for the local area)
Heart FM South Coast transmits on these frequencies:
102.3 Medium power (2000 watts) from Nine Barrow Down (for the Bournemouth area)
97.5 Low power (850 watts) from Fort Widley (for the Portsmouth area)
96.7 Low power (500 watts) from Crabwood Farm (for the Winchester area)
Chris