A study was undertaken to estimate the radial diffusion timescale, τLL, for relativistic electrons (2--6 MeV) to diffuse into the slot region due to drift-resonance with Pc5 ULF waves (2--10 mHz) on 29 October 2003. Large amplitude ULF waves were observed by ground-based magnetometer arrays to penetrate deep into the slot region (L $simeq$ 2--3) starting at 0600 UT and maximising (~200 nT p-p) between 0930--1630 UT. Around the same time, the SAMPEX PET instrument measured an over two orders of magnitude increase in relativistic (2--6 MeV) electron flux levels in ~24 hours within the slot region. The ground-based D-component magnetic power spectral densities (PSDδB) for 29 October were estimated for six latitudinally spaced ground stations covering L ~ 2.3--4.3 for an observed ULF wave with central frequency ~4 mHz. The PSDδB values were used to calculate the in situ equatorial poloidal wave electric field power spectral densities (PSDδEm) using a standing Alfv¿n wave model. The radial diffusion coefficients, DLL, were estimated using the PSDδEm values. The fastest τLL were 3--5 hours at L > 4, while τLL initially increased with decreasing L-value below L $simeq$ 4; peaking at L $simeq$ 3 with τLL ~ 12--24 hours with PSDδEm estimated using a wave frequency bandwidth between Δf = 1 mHz and Δf = 2.5 mHz. The τLL over the L-range L ~ 2.3--3.3 were consistent with the timescales observed by SAMPEX for the increase in relativistic fluxes in the slot region on 29 October. The authors believe that this is the first example of the ULF wave drift-resonance with relativistic electrons explaining a radiation belt slot region filling event.