I have an HDR-1800T which came with
Model number: SH1215WWSA
Humax Part number: SH1215WWSA1A
DC Output 12V, 1.5 Amps, 18W Max
AC Input 100-240V, 0.9A
If yours is a 500GB it may need at least a 2.0 Amps rating rather than 1.5 Amps.
The following boxes I know for certain have power adapters which would suit a HDR-1800T (but not necessarily the other way round as the HDR-1800T power plug may have a slightly too low amp rating for them).
DTR-T4000
DTR-T2100
DTR-T2110
DTR-T2120
HDR-2000T
Virgin V6 TiVo
(There are others which also look compatible but its only those models i have listed above which I directly know.)
A quick internet search and I found that seller DIXIETREE lists two of these models as
5.0mm x 3.3mm with centre pin.
I then searched for the Virgin V6 adapter I have which I know is compatible (PE-1300-9AR3) and came up with a couple of different sellers that also specify it as 5.0mm x 3.3mm