Your DNS records are
OK (using Google's DNS servers) I'd normally use a CNAME for the www subdomain, but that's not critical.
tstdb.co.uk. 14177 IN NS ns.123-reg.co.uk.
tstdb.co.uk. 14177 IN MX 10 mx0.123-reg.co.uk.
tstdb.co.uk. 14177 IN SOA ns.123-reg.co.uk. hostmaster.tstdb.co.uk. 2016010402 14400 0 604800 14400
tstdb.co.uk. 14177 IN A 95.142.152.194
tstdb.co.uk. 14177 IN MX 20 mx1.123-reg.co.uk.
tstdb.co.uk. 14177 IN NS ns2.123-reg.co.uk.
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Reverse lookup fails - but that's not a serious problem
If I use international DNS some places can't resolve your address (e.g. Canada) but that's probably just the result of slow propagation.
Check your server logs and please post any errors, and your
.htaccess if there are no errors in your webserver logs.